I think there's a good chance you may have found the problem.

Same happened to my 12S. Wondered why the fuse kept blowing? Fitted new one (from Halfords) & changed over cover to keep LR logo (PITA). Previous cars:
S-Max 2007-2013 (only diesel I've had, good car but expensive when diesely bits go wrong, so what's the point?)
Galaxy 2001-2007
Mondeo Estate 1997-2001
Sierra Estate 1993-1997
Uno Turbo 1987 -1993
Fiesta 1984 - 1987
Fiat 127 1982 - 1984

Well, went to come home from work and the thing won't start again. I even tried the booster they use for vans and while it did try there wasn't enough to start. The volt meter went down to 10. Something then back up in the 11's

I love this car but if I can't rely on it it's going to have to go

Back to the drawing board.

Well I left it overnight then got the jump leads on it. At first it wouldn't start though the voltmeter was reading 13.5v but after a couple of minutes it fired up.

I drove home which is only 3 miles and parked it up but didn't put it on charge right away. About an hour later I went to it was showing 12.3v and started first time. I wasn't expecting this as I hadn't charged it or driven if far and am a bit puzzled.

I have been reading posts about second batteries and am contemplating this to give me some peace of mind as I'm getting paranoid about being stranded. Reading the posts though there seem to be differing opinions. For my purposes can I just fit it in the boot and connect to the wiring and leave it at that?

Iirc the boot wiring wouldn't be up to the job to provide cranking power for the starter.

The problem you will face is if one battery is slightly lower than the other then it will pull the good one down and keep doing so until they are equal or the better becomes depleted.

I'm starting to wonder if the cold crank ability of your battery is shot. This is very hard to test for and something I have come across myself recently on an old big engine diesel that requires a good battery to spin it over or it just goes slow turning over.

The battery had good voltage at 12.8 and the new one was showing 13 but when the new one was put in it down straight over and the .2v difference wouldn't factor into it really.
The car has been as good as gold since as well doing cold starts after a week of sitting even in freezing temps and running non standard fuel which also makes starting harder as it taxes the starter and the glow plug system  FFRR MY06 facelift With TDV8 Alloys Zeros/ATR's
Mantec Sump Guard, Rigid Load liner, MY10 BT upgrade.

The CCA capacity of a battery is quite subjective and there are several different ways of measuring it. It should be noted that the standard LR battery made by varta in the tdv8 has a higher CCa rating than some of the larger Bosch and carts silver batteries we tend to fit as replacements, buthey I'm not sure that they are measured in the same way........ There is nothing that can't be fixed with a hammer😜😜
FFRR 4.4 SDV8 Autobiography Santorini Black.
KIA E-Niro 4+
2021 Discovery 5 D300 MHEV commercial

Testing CCA isn't always a reliable test from what I can see unless you repeat the test a lot of times to get a good average or just fully change it off the vehicle and compare to another one that's brand new...

I tested and tested mine when it was giving issues and charged it off the car on the regen and it was still the same.
Stuck a new battery on and job done  FFRR MY06 facelift With TDV8 Alloys Zeros/ATR's
Mantec Sump Guard, Rigid Load liner, MY10 BT upgrade.

Do you have a voltmeter you could connect permanently inside the car so you could check the voltage whilst you are driving?

Can do that via the hidden menu on the sat Nav screen

Get to the home screen, press the top middle of the screen (no obvious button) for 10 sec, then press and hold the Nav button on the right until the keypad pops up. Enter 753 ok. Then press the vehicle signals button. Voltage is top left. There is nothing that can't be fixed with a hammer😜😜
FFRR 4.4 SDV8 Autobiography Santorini Black.
KIA E-Niro 4+
2021 Discovery 5 D300 MHEV commercial

Well there you go You should be able to monitor the alternator output as you drive. If it's consistently above 14.0V it's not your problem.

Mine goes high as it puts the voltage back used to start the car, then settles around 13.5-14v the built in volt meter only steps in .5v so I'm assuming that's 13.8v which is bang on.  There is nothing that can't be fixed with a hammer😜😜
FFRR 4.4 SDV8 Autobiography Santorini Black.
KIA E-Niro 4+
2021 Discovery 5 D300 MHEV commercial

Given the inbuilt voltmeter reads on 0.5V steps I'd be inclined initially to use a possibly more accurate voltmeter initially.
But that just me.

I have one of those that plugs into the 12v socket that someone here recommended. All readings are as expected. I now have it plugged in the boot socket with the display on the parcel shelf so I can see it without opening the car.

I gave it a short run yesterday and it started no problem again this morning. Voltmeter was showing 12.2v.

My concern now is if I have a few days in the office it's going to let me down again and that's why I was thinks of the 2nd battery. I can carry jump leads but will a small hatch have enough to jump it?

In my experience the only good cars to jump off are 2.0diesels as I've tried before with a 1.6 golf and it took a few minutes more to get enough juice in compared to a 2.0 gtd and some 2.0 Citroens I've had around  FFRR MY06 facelift With TDV8 Alloys Zeros/ATR's
Mantec Sump Guard, Rigid Load liner, MY10 BT upgrade.

12.2V is low if it stood all night prior to starting.
Nothing wrong with carrying jump leads, it is a LR after all.
Almost any car will jump start a RR as long as the cars are connected correctly and the "donor" car has the engine running at a reasonable rate. Just give the donor car a few of minutes running and then start the RR.