Puma Boiler - overheating??

I have a Puma Potterton combi boiler and today the lockout red light ame on. I've tried resetting and this fires back up ok. It seems to be sufficient to run the hot water but with the central heating on the boiler seems to get hot and hisses (gage shows over 3bar) and I don't know how to lower it - it then locks out again. The service man came a couple of days ago while we were on holiday and I'm suspicicous that he's done something to upset it. Any ideas?