Hot n cold

You may remember my topic regarding my heater constantly blowing hot and my aircon seeming not to work?

After a few replies and looking at old posts, I decided it could be the heater valve body located near the nearside against the bulkhead.

I noticed the other morning though (the first cold morning for a few months) that the heater was blowing cold. I have never bothered changing my heater to hot because it is always hot anyway, but on this particular morning, having the heater sitting on cold (which i always do anyway) it did actually do what it says on the tin! it blew cold for once! So surely it cannot be the heater valves sticking? My theory is possibly the cabin temp. sensors? (for the climate control) If they play up, does the heater stay hot? Is there any way to test them?  I am still going to give the heater valves a good clean but I'm not sure this is the problem.

Any help greatly appreciated!

Mike