Boiler Ignition lockouts. Potterton Promax 15HE

I posted this to uk.d-i-y during the week but no response so try here.
Boiler now seems to lock out but only after is has reached it operating set-point. That is it doesn't lock out if either of the remote thermostats (cylinder and room) switch it off first. The CORGI registered installer/maintainer (it's four years old) has now replaced the igniter and sensor and both the main control PCB and the Ignition PCB have been swapped out and so we are pretty sure it is none of those. I'm wondering if the safety cut-out is operating before the setpoint is reached.. Perhaps we should start measuring the temperatures. I guess by closing down all the TRVs we could speed the process up a bit. He's loathe to hang around for the hour and a half it normally takes. Or could the control thermister not be functioning in and allowing the boiler to overheat. It's currently set to operate at the factory set point of 82 degrees. If anyone else has had this or similar problems I'd like to hear their solution or suggestions.. Michael
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