Fixed it!
Tried to get the 'stat out of the housing, snapped a TORX bit and so didn't actually get it apart... but gave the system a good flush, ran with some Boi washing powder in to clean the crud out, and also got some needle-nose pliers up the oil cooler outlet to give the end of the 'stat a bit of a free-up.
All back together and working perfectly, bottom pipe hot now and no overheating
Worth a shot, took me 30 mins!