Vokera 28e Hot Water Problem?

Have had the combi boiler for 10 years now with few problems but just recently I am having intermittent problems getting hot water. For some reason the boiler will not ignite. I have tried putting the heating on and this sometimes works and gets the hot water flowing, and usually after a few hours (4+) I can get hot water again. I have had it serviced regularly and have had engineers out 3 times to look at this problem but by the time they get to me the thing has righted itself.
So..........problem still not sorted. Can anyone throw any light on this? Pleeease!