Rogers comments made me remember the 'stat mods I use on Mini's (may be worth a try Neil as 'stats are so cheap) I drill 3 x 1/8" holes in the baseplate so they constantly bleed some water flow
You just reminded me Jon of a similar mod I made to a vintage narrowboat engines thermostat. Here I drilled a 2mm hole which aided bleeding as well as providing a bypass trickle. If the stat has a horizontal component to its mounted position the hole needs to be at the highest point.
Thinking further about Neil's issue it may be the best answer to his overheating. I would cautiously start with maybe just one say 2-3mm bypass hole in the periphery of the stat and see how the engine responds. Note stats are usually manufactured with a bleed hole anyway, partially filled with a captive jiggle pin to aid same.
