Sounds like pressure relief valve isn't sealing - either dirt or needs a new one. Try opening it and letting some pressure away to clear muck and then re-pressure.
Don't worry Phill he's not typical of most plumbers - they're quietly much worse but certainly haven't got much time for your job as they have to be off planning what to do with their money - no only kidding some of my best mates aren't plumbers. Yes that is the law new boilers fitted must be...