Jim, all very valid points. Here are some of my thoughts on it all:

My issue was different from hill mode. There are plenty hills around me, and when in hill mode, it holds a lower gear and won't shift up till about 3000rpm instead of 2500rpm ( or whatever ), after cresting the hill, it holds the gear for a few seconds, realises the demand is gone and then drops back into normal mode. MY issue was different. IT was holding the lower gears for a few miles, not just seconds. The box was definitely doing something different. IT reverted back after a few miles of me backing right off and driving gently.

As for the thermostat:

By design, the engine can't run at 105degC all the time. If it could, it wouldn't need the electric heating element in the thermostat to force temperature down to the 80 to 90degC mark ... which implies that for heavy load running, the engine should be worked in the 80 to 90degC range and not 105degC.

In fact, the way I see personally it is that the engine should run at the 80to90 range and is being artificially held high at 105degC just as means to reduce emissions.

under general driving ( and no diagnostics attached ):
How do I know if the ECU thinks that its high load conditions? I don't.
How do I know if the ECU is trying to force open the stat? I don't
How do I know if the electric heating element is working? I don't.
How do I know that when towing, the engine is being cooled properly? I don't.

Lowering the stat temp to me simply ensures that i'm always cooling the engine to bring it into a temp range where it can run at ALL loads at ALL times. Yes, I may potentially lose a little performance/efficiency but at 13 to 14mpg when towing who frankly cares.

So I'm not running it out of spec per say, I'm just trying to run it at the other end of its normal operating temperature envelope!

As for transmission cooling ... yeah, considering that too ... there is a ton of information on the web that high temp greatly degrades transmission oil and that thermal protection of it goes a long long way to the reliability of a gearbox. Having already shelled out £££ on a rebuild, I really would like that reliability.

The lower stat temp may well be plenty to cool the transmission as well, so I'll just try that first.

I'm not crazy, its the other voices in my head that are ! .... wibble.  V8 or else ...

It will be interesting to see if it makes a difference!

I assume you have already checked the obvious? Rad free flowing, water pump impellor good etc? And, trailer brakes not binding?

Simes,

Its not a problem I get on a day today basis, its just something I've had happen a couple of times. Its more a general thing where I finish a trip in the car, walk away and the engine is reekin hot. It smells like an over cooked engine. Even the wife comments on it.

Water pump was replaced recently.
All the pipes have been placed recently
Radiator has been replaced in the last few years.
When filling up the system, its clear in the expansion tank that there is a good flow of coolant through the system.

but

I do have a small leak somewhere ... beginning to think be the radiator as the coolant is visible on the fan blades. Will need more investigation further down the line. V8 or else ...

Hmmmm...

I can see what your problem is - and it's not 'normal' which is what makes me think there is a problem elsewhere. I hope I'm wrong!

I think it's a great idea, il be watching this closely one thing I'm not happy with is the pressure as a result of running so hot all the time, I really think the rad wouldn't fail so so easy if it was running cooler, on an older car surely dropping the temp and relieving the pressure a little has got to be a good thing, and like you say it's still within the engines tolerance, just towards the bottom instead of the top

What weight are you towing? I regularly tow around 2000kg plus a load of tools and things in the back of the car - probably another 200kg's, and never get out of it with it smelling like it has been struggling and I've never had the gearbox overheat or anything. That is obviously quite a way below the 3.5t limit stated in the manual.

Are you certain the radiator isn't blocked either on the outside, the inside or both.
Usual reason for engines of any type over-heating.

I took an airline to my radiator last year, blowing from the engine side to out the front of the vehicle, amazing the amount of dirt that came out of it

I think i need to clarify a couple of points here:

I don't think my engine has overheated or is overheating ... the needle has never moved above halfway. I think that it just runs hotter than what i'm comfortable with for my uses. I think I work my engine fairly hard sometimes and its those times that are more important and I want to know for sure that the coolant is being kept at the lower temp range that the electric heater would try and aim for ( assuming its working )

I think the small coolant leak ( where ever it is ) exacerbates the matter cause after a hard run, I can smell the hot coolant.,and that compared to other engines I've had, it all just smells and feels that little bit hotter than the V8's that have preceded it. Thats what I mean by reekin hot.

So I'm not looking to resolve a problem with the cooling system such as a blockage ... I'm just wanting the system to run at the slightly lower temp and pressure and get the benefits and peace of mind from that. If I thought I was over heating coolant then absolutely I would be looking at blockages and similar issues ... but I don't have them.

In terms of the transmission issue, its only happened a few times , but when it does, I just get paranoid about ruining the gearbox. Lower coolant = better cooling of transmission = peace of mind

Again,its not a case of whether fluid is being cooled or not, its whether its being cooled sufficiently. V8 or else ...

Well, the part arrived today. visually, its a match. so it should fit and it clearly says 88degC on it. all good so far.

In fact its sooooooo much of a match, that despite being in a Vemo box, the casting says BEHR on it, with the same date dots of manufacture as the old one i have on the desk, and all the small crackle tooling marks on sections of it are also an identical match. Me thinks someone has been copying parts I'll post some pictures later on when I get the chance.

What I will also do, is record a run in the car showing engine temp, transmission temp and hopefully throttle position. The idea being going up a road near me trying to maintain 30mph up various gradients ... that way, the loading on the engine remains similar across both runs.

I'll do that with the 105degC stat and then try and repeat it with the 88degC stat along the same run.

Hopefully that should show some reasonably comparable temperatures. V8 or else ...

interesting topic ,
my experience of this is , my gearbox would hold in lower gears after driving 30-40 mins , i never had transmission failsafe as i belive you wont get this untill the gearbox fluid reaches 130-135 deg c as my gearbox was reching 125-128 deg c it seemed the gearbox ecu was trying to protect the fluid by increasing engine rpm to cool the fluid faster , i measured the geabox with a lazer temp thermometer , and taking several measurements all over the engine and gearbox and recorded the values , i then found the radiator to be at fault so i changed it and the result was perfect , however as this being a cooling related problem i changed the gearbox fluid , the gearbox fluid cooler ,all the engine cooling pipes , water pump and thermostat for piece of mind and all is fine , remeasured the temps and found a signifiicant temp difference on the gearbox and related parts
but the twist in this diagnosis is that the engine does run hotter because when i removed the thermostat housing
i found the stat was missing thus cooling all the time i never had any faults as a result of this just a cooler engine , the temp allways went to half way but just felt cooler
i replaced the whole stat and housing the engine reaches the half way on the temp gauge as before but you can sense the engine is hotter , i did not notice any mpg difference as a result of running with or without the stat just a cooler engine , why do these engines run so hot ?
i am seriously thinking about removing the stat alltogether and run a cooler engine  1990 3.5 v8 defender
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I promised some pictures of the stats:

Old 105°C BEHR on the left, new VEMO 88°C on the right:

See some similarities?

Click image to enlarge

Click image to enlarge

Click image to enlarge

Click image to enlarge

Well, quick update. I've simply not had time to do a run to record the old stat temperatures.

However, the new stat is fitted this morning. Took about 10 mins to unbolt and replace. Its a perfect fit.

On its first run:
- The temperature gauge on the dash remains bang in the middle like it used to.
- No errors on dash from leaving the heater plug unconnected
- The hot air into the cabin is still toasty hot.

I'll plug the fault mate in later on and see what temp it runs at now, but overall, I'm super happy.

Its early days, but feel like I can drive it and work it with peace of mind now. V8 or else ...

It's good to see it's OK and with no errors - but I still feel you are trying to fix a fault that doesn't exist.

I work my FF hard enough with the caravan or horsebox on the back and have never had a moments concern about it overheating and no smell of overheating from under the bonnet...

Your right, there is no 'fault' as such.

Consider it a modification . V8 or else ...