How can I stop my Potterton 80e from losing water and pressure?

My 80e loses pressure as soon as the central heating is run, have fitted a new pressure rlief valve. Pressure goes higher than 2 bar when heating is running, I then get a lockout and have to isolate the sensor for about 6 hours before I can reconnect it (because the lockout light stays on after I top the system up if I reconnect it). 2 different issues and I assume the latter is because the sensor needs cleaning up/replacing but it's the first problem I 'm really concerned about.