vaillant turbomax plus is not responding to settings?

Hi just wondering if any one has an idea why my vaillant turbomax plus is not responding to the set settings.
1. CH is on timer, switches on at sett time does not switch of at set time, I then have to do it manually, it still does not respond on many occasions I then have to reset the whole system, switch it off and on again and eventually get a result.
2. When CH is on no hot water comes in the tap.
3. After CH is turned off hot water tap is turned on the CH start heating up very powerfully but no hot water.
Some peculiar things happening, I have contacted a plumber who has suggested that the divertor valve needs replacing and that should do the trick, it will cost me around 125 quid is this a right diagnosis and is this the general costing to replace the valve, happy for any response.