The parts were very easy to get. All the wiring, connectors, even the fuse was already in my Mégane - all I needed were the four heated pads, two switches and new trim for around the handbrake with the holes for the switches in. I got the pads for free (should be £100 each - my mate was breaking a Mégane) and paid £42 for the two switches and trim from Renault - they arrive on Tuesday.
All in all it took around two hours, and was much easier than I expected. If anyone wants any advice or pointers just give me a shout!