im looking for a bit of general advice, my rr is quite new to me, so im still sussing out the foibles.
the latest one 9for now and I know there will be more) is that when I put on the heated seats, the lights come on but they don't seem very hot at all.

Question is:
Do the heated seats heat up to a certain temperature, i.e. could it be that they work but it wasnt nocieable due to it being a hot day when I tried them out?

they have 2 heat settings, press twice for max heat. They do get pretty warm after a few minutes and you will definitely notice it ! 2006 RRS SC Bonatti Grey (the 55th car/van I have owned)
2005 GSX 1400 K5

hmmm.. that tells me that something is not quite right then, as i did this and they just dont seem very warm at all.

Is there a particular fuse that may need replacing, or would it be the case that if the lights are on then the seat should be warm, i.e. both work or not at all?

They tend to get hot pretty quick when the engines running.
Unless you have a blown fuse, check the fuse , it could be just a simple fix as a fuse.

Both seats? ON THE WAY: BMW F21 M140i
MERCEDES GL350 - Metallic Navy Blue w/Grey interior

GOING: BMW F31 335D MSport+ xDrive Touring
GONE: Land Rover Discovery 4 HSE - Stornaway Grey w/Black Interior (wish we'd kept it )
GONE: BMW F10 520D MSport - Carbon Black
GONE:MY12 RRS SDV6 HSE Sumatra/Ivory/Grand Black Privacy glass

Seeing that your vehicle is a TDV8, I would get your battery and alternator checked. It could be early signs of one of them failing. Common fault on early TDV8.