When connecting and pulling tight the cable for the thermostat I am pulling hard against the end of the thermostat and it seems the force is pulling the thermostat to it's length!! Is it now trashed or was it trashed and this is why it is doing this? I have read all the popular advice on the web and it doesn't seem to address this....

I would say it was trashed when you were doing the cable. Thermostats are in very short supply if not NLA. Someone is making do by modifying Type 1 thermostats; best solution I think short of opening the flaps by a hand lever (friend's car like that)

PS: I was curious about where Prescot is so I Mapquest'ed it. My son lives in St Helens while attending John Moore U in Liverpool.

I have a spare....so what is suggested as a method so as not to strain it??

no method that I have seen. Even factory workshop manual just says to hold flaps open and tighten cable (3.3-2/4)

Don't pull on the cable. Just take out the slack and pull the clamp against its spring and tighten clamp to the cable.

awesomepowdercoat.com
I found this guy. Bought a T4 stat from him. Seemed like a reasonable price for a NLA part and he has the T4 with the correct operating temp. I even tested mine. Had to, my engine does not get very hot sitting on the table like it is. Anyway, it is also a great resource for how (why) the T-stat works. And his name is the same as mine. Not many Clarks around.

Buy one and end the worrying.

Clark

Mine works well. Buy new cable with it.