Fuse keeps popping on central heating system?

This morning my central heating fuse blew and i initially thought it was the timer / programmer so i bought a new one. The fuse is still blowing with this new timer / programmer. I have been told that it could be my back boiler that could be at fault, would this be a possibility? or what things can i check to find the fault. Any help would be great.

jamie
May 2009