Finally, a good news update here:
After a lot of searching and testing etc, it transpires that I did in fact buy a heated wheel for a 2014> Discovery which as was suggested requires a CAN signal and runs on a faster CANBUS than the earlier models.
The earlier D4 actually uses the same heater ECU as the L322
BUT the wiring is different lengths and you can't easily chop the wiring about based on the wiring diagram in this thread as it shows 4x red wires that don't exist! Also, the switch packs can be different between the two iterations making it very hard to cobble something together..
In sheer frustration, I broadened my search for parts as these heated wheels are rare to say the least. It appears that most US versions of the Range Rover Sport were fitted with heated steering wheels which meant that I was able to buy a wheel from one of those and had it shipped from Illinois (the irony of importing car parts from the US for a Land Rover is not lost on me!).
The RRS wheel of course did not have CommandShift paddles but it did have the ECU and switchbacks that I needed so upon arrival, I stripped it down, removed the heater ECU and back cover (which is different to the 2014 part as the ECU is a different shape) then I fitted that to the wheel I had bought, swapped the Piano Black switch covers and built up the wheel. 10 minutes later it was in her car and heating up beautifully.
Claire is absolutely thrilled with this as she has trouble with blood circulation to her fingers making them extremely painful when they are cold. As they say, happy spouse, happy house!
Now, I have a few bits left over!
Having fully cleaned the RRS wheel, it might actually be nicer than the D4 wheel I got originally but I'm not changing it all over again so I've built that as a 2014> D4 wheel and the original D4 wheel has been put together as a base wheel for those without command shift.
Does anybody want to make an offer on a heated wheel.... Please?
I'll list in the for sale section and stick it on eBay shortly I expect.
Thanks to everyone that chipped in to help out on this, we got there in the end!