Vaillant Combi VC242EH switch for hot water not clicking in?

I have a Vaillant Combi boiler 242EH but cannot get ant hot water through, without removing fron lower panel off boiler & manually pushing over the cam to operate the micro switch (with hot tap running). I'm thinking there must be a diaphram in the hot water change over valve that must be perished any ideas please...
What parts would I need to order & roughly how will they cost?
Will be repaired by a gas engineer (just need an idea 1st)
Many thanks in advance