Oil boiler will not start first thing but is OK rest of day?

After being off overnight the oil fired boiler does not start in the morning - the pump comes on but the burner doesn't light. A small, electrically lit warning lamp housing a push button lights up and, on pushing the button, the boiler then starts. It is then OK for the rest of the day only for the problem to re-occur the next morning - any ideas?
The boiler is a Myson but I cannot see any model number etc