Worcester 350 combi not heating domestic hot water?

I have a worcester 350 combi that works ok on the central heating side, but randomly fails to light when drawing hot water, There's no spark but the pump always runs. There are a couple of sensors on the outlet from the heat exchanger that I imagine are a temp and flow sensor, that seem like candidates for repacement. Any ideas gratefully received.

brianh
September 2008