1.9 non turbo diesel (1996 estate)

I posted a message some time ago  about "STOP" light not working.

Shortly  after I posted message about dash lights then they started working
80% on start up (After oil change and a bit of Gunk spray clean up on engine
block may have caused this - who
knows? Weather as been very hot as well)

Bit worried now as the "STOP" light comes on with the temp warning light at
irregular intervals when the engine is running and up to normal temp - in
fact showing 80C. (Peugeot handbook states 90c for normal temp) Fan operates
when at 80c and even when engine switched off /ignition on, will operate
down to under 70c.

It does switch off at a lower temp as I waited for 15 minutes and the fan
did start but stopped after a few minutes. Unlike my earlier 405 it does not
run on for 12 minutes after ignition switch off.

Could this just be a faulty sensor/faulty temp gauge or maybe water pump
problem? Rad water
level is ok, no sign of overheating,  and no cap lift off to discharge
water. No sign of bubbles in water etc.

Are the temp sensors colour coded? where on engine block?

Would it be prudent to have a compression test just to see if the head
gasket is going - (New head gasket and only done 7000 miles since renewal)
No sign of any oil leakage.

Thanks for your help