Vokera Excell 80sp hot water fault?

The heating ignites as it should, and works fine, but when we only want to get hot water(dual heating and hot water option on), when we turn the tap on, the boiler does not ignite. We can trick the boiler to get hot water when the heating is on, when the the boiler has ignited, but this is intermittent (ie water comes out warm, then hot, then luke warm etc).

Any thoughts on cause of problem? Some dodgy switch somewhere?