Thanks namsag. I still couldn't find any way of getting the cover off, but your junction box approach has done the job for the moment. I now have heat and my sanity :)
Yes, sorry, I meant the programmer. I need to remove the cover because that's where the lead from the valve goes.
I was under the impression from my neighbour that all the heating equipment in these houses was put in about 22 years ago - which I thought would count as old :)
I need to change the head on my motorised valve, and I'm falling at the first hurdle.
I have a very old Thorn EMI heating controller, which I obviously need to get into to get to the wiring, but for the life of me I can't work out how to get the cover off! There are no screws that I can see...