Ok, thanks Moody1. I thought 74 degrees was low!! I bought the thermostat from ASAP supplies, so first port of call will be them - bet I know what the answer will be though. Only problem I find with Perkins info is that you have to go to local agent who then contacts Sabre ( I think it is) who then contacts Perkins and lethargy takes over in my experience. At the temperatures you quote I may as well remove thermostat all together.

Any chance of a scanned copy of the manual you have?

I'll try and remove pressure relief valve - despite temperature I noted on gauge (which may not be particularly accurate) this relief valve did not open. As I do refrigeration work, measuring temperatures accurately is not a problem.

Thanks everyone for the comments.