Only get hot water when heating is on with Vaillant Combi boiler?

I am unable to get hot water when switch is set to domestic hot water. However I am able to get hot water when switch is set to domestic and heating. Guy in the office says it sounds like a valve is stuck, more than likely the central heating pump valve. He says sometimes a gentle whack can free the stuck valve. Problem is I don't know where to find the bit I need to give a so called 'Love Tap' too. Is he correct and any further advice please.