Danesmore 26/32 oil fired central heating boiler keeps switching off?

recently had boiler seviced. all was fine until now boiler keeps switching off. the flame out light comes on but when button is pushed on the boiler it lights up runs for 5 mins and then switches off with the flame out light back on, the heating engineer cannot find anything wrong with the system please help.the boiler is 16years old and up to now has run perfectly the boiler is a Worcester Danesmore 26/32 oil fired central heating boiler