Worcester Heatslave 20/25 5 amp fuse blows?

My old Worcester oil boiler has started blowing the small 5 amp fast fuse.If I isolate the wiring going to the control box,it doesn't blow,so I thought it must be the control box,so bought a new one,but it's still doing it.I'm thinking it is something to do with the burner,or the various parts that have wiring coming from them,but I don't know how/what to check.It's going to be -10 degrees tonight,so any help will be gratefully received....