valliant boiler?

hi and thanks for taking the time to answer my question.... we cannot get any hot water for bath or shower or any taps untill we switch the thermoostat on the wall on to about 21 -23 then the little flame display comes on , it then sends the lights on the boiler on all except the shower symbol, we then get hot water in the tapes , we have been told it is working of the central heating...... someone as told us its either the diaphram or the diverta valve, is this an easy job to do or will we need an expert thanks mick.....