New Potterton Gold Combi HE 28 intermittant hot water

[texastut]

2 weeks ago I have a New Potterton Gold Combi HE 28 boiler installed. It is great for heating the rads, very quick to reach temperature and always works. The hot water output however is intermittant with no obvious reason. A potterton engineer visited and could find no fault and it was ok everytime he turned the tap on. Later the same day, no hot water again. The indicator light does not come on when this happens. It can happen if the heating is on or off.
Any help would be appreciated.

 

[Vic Spanner]

1. Hall effect sensor/Diverter valve may be faulty. With the case off, next time hot water does not come on ,see if the sensor lights up ..Both can be quickly tested
2.Out of interest did you have a shower fitted at the same time

 

[texastut]

No new shower fitted, there was a problem with the old stopcock though, which was replaced with the boiler.