How can I fix my Horstmann Economy 7 Quartz/Maxistore boiler?

My boiler sporadically stopped working last week, so I checked the main fuse - it had blown out so i then replaced it and turned the system back on... the light turned on and it appeared to be heating the water so i didn't worry about it. About 2/3 hours later I heard a snapping sound from the fuse and a thud from the boiler itself, the main fuse had blown again so i replaced it again, and the boiler turned itself on and then fused out 2/3 hours later. I think the problem is something to do with the control timer box, as if it can't cope with anything over a certain temperature - it's been fine for almost 3 years now... Any ideas?

The control box is a Horstmann Economy 7 Quartz and it says 'Maxistore' on it (picture here http://www.alertelectrical.com/product.asp?typeID=164&subID=0&prodID=3877)

N Davidson
April 2006