CENTRAL HEATING CONTROL SHORTS OUT?

I have a Mynute 14-48 SE central heating boiler, controlled by a Drayton ACL LP112 controller.
The LCD display is blank. I changed the 3A fuse adjacent to the controller, the LCD display re-appeared. I tried to switch the central heating on, and the display went blank again. Fitted another 3A fuse, display re-appeared. This time I tried to re-set the time, and as soon as I pushed a button, the display disappeared again. The third time I replaced the fuse, I left the unit alone, to see what would happen when the set time cut in; it cut out.

Clearly as soon as any demand is put on the unit it shorts out. The central heating system itself was serviced over a year ago, when the problem was the impeller shedding its blades. All defective units were replaced. None of the other switches or valves have been touched since. Before the last service the unit worked fine for over 4 years.