my potterton 10/16 1amp fuse blows as soon as boiler ignights?

my potterton 10/16 1amp fuse blows as soon as boiler ignights. this has happend before.ON returning home on holiday six months ago it happend engineer put new fuse in and it was okfor six months.cost 35.00.I done the same and fuse blows as soon as boiler ignights.i take two screws out of fan cover to check the fan and i find the third screw has been rounded of byprevious engineer.Am i wasteing my time checking the fan.What are the possable causes of this problem.JOHN