Thermostat fitted this morning
I had tested the new one from Shop4parts before I fitted it. My infra red thermometer showed it opening at roughly 85 - 88 c and it closed properly when cooled.
Fitting the new one was pretty straightforward although one of the cross head bolts holding the black wires box in place was a bit stubborn. Some Plusgas (which is a much better penetrating fluid than WD40) and a few taps on the screwdriver with a BFH helped free it.
I did a test drive through town and at speed on a dual carriageway. The gauge gets up to about 80 in a few miles and then sits there at all other speeds until back in town and crawling/queuing when it goes to 90 before the fans kick in. Result!
The other plus is the toasty air now coming from the heater
Red Spider S4 LHD manual 1990, Silver 916 V6 Spider