worcester 24i combi overheating?

My 11 year old worcester 241 combi has just had a new pressure reservoir fitted. since then the boiler trips out the over heat button after 15mins. The engineer has taken off the bathroom rad stat to create a bypass but that has not solved the problem. He thinks the narrow bypass pipe in the boiler is blocked. What should he do now? He has mentioned having a room stat but that feels like a backward step to me.