The other morning, my battery was completely flat. I fitted a new FSR as suggested before anything else and charged the battery. We had to use the car so only had a couple of hours charging although she started up fine. On the trip my wife commented the heated seat was not so warm and I noticed the car didn't seem to dehumidify and was really steamed up although the a/c light was on.

I have the battery on charge now and will leave it for a few hours. My question is: does the car, when the battery has a low charge, switch off various electrical items or reduce the power to them ? Is it intelligent in that way I wonder or do I have some other issues ?

FSR?

It sounds as though the alternator is not giving the power you need. High drain items like heated seats and heated windows. However, AFAIK, aircon is run off a compressor driven by the engine, not the electrical system. So there is a belt that drives it, like the one that drives the alternator (someone else can tell you if they are the same one - unlikely but possible.

It might be that one / all of these belts are slipping. Any screeching noises?

I would be inclined to check the voltage on the car with the engine running at a fast idle.

After giving the battery a full charge, the light flashes & stays on intermittently during a 20 mile journey

Upon returning, the batt was reading 10.5 v engine off and couldn't get more than 11v engine running at various speeds

Would that mean the batt is ok but not getting charged ?

You have an alternator problem which means it either needs refurbished or replaced.
Your battery may well be ok just not receiving an adequate charge, it is possible however running it at low voltage could have ruined it. I assume when you charged it up it held a charge sufficient to start the car in which case you may be lucky and it's still ok. You could get it tested before doing anything about it.

Thanks for your prompt reply. So a new alternator. Any recommendations ? I seem to remember reading before it's an expensive part & a pia to fit

Exactly the same happened to me last weekend

New alternator, fitted by a good independent - just under £500 but that was a TDV8

Provided your car vin is below 5A999999 this will do you, unless you want a new genuine one at about £900.
Might be worth asking Dan at Duckworks the price of an aftermarket unit, he usually does good prices.

http://www.rimmerbros.co.uk/Item--i-LR005866P

Wow - and I thought the TDV8 was expensive

Right enough a new alternator has sorted everything. Squeezed as much juice into the battery as possible overnight & set off to my indie 70 miles away for fitment. 20 miles in, the dreaded bong & lots of warning lights, stuck in 4th gear & couldn't risk a restart to reset. No heating, no lights, I even turned off the radio. Never thought I was gonna make it. It just shows how the car will keep the engine running but shut down everything else. On the return journey I noticed the seats & steering wheel toasty hot, wipers faster, a/c demisting properly and everything feeling good again.

well done fella