I had exactly the same problem. Potterton profile 50e The heating would not start on its own. However if you put the water on the heating would start (and then only till the water tank reached it.s optimum temp. I fixed it last night by changeing the motorised valve that sits on the pipe (mine was a Sunvic 3port) - The part cost £36 quid from B&Q simple to fit - although it came with the big brass valve - i only used the motor (white bit). Hope this helps
Steve Kumar (CCNA MCSE) not (Corgi) lol
Steve Kumar CCNA MSCE MA
November 2007