Burner will not ignite on our Worcester Greenstar 12Ri Boiler ?

Our boiler has been perfect, I re-programmed the timer to come on for less time whilst we went away on holiday, when we come back the house was freezing cold. When we pressed the main module to switch the heating on constant we noticed the burner light was not illuminating, the only way we could get the heating on was to press override on the hot water which got the burner to ignite... This was ok for a day, now we can't even get the burner to ignite for the hot water, is there an easy fix for this or is it an engineer fix... Just to add we have the main boiler light lit constantly it's just the burner light that's not operating... Thanx in advance