Hi,
I'm a bit of a novice so please bear with me.

The fuse in the fusebox next to my central heating controller keeps
blowing. I replaced it twice yesterday with a 3A fuse and again this
morning with a 5A fuse and within half an hour of the heating coming
on and the radiators just starting to get warm it blows.

Has anyone got any ideas as to what the most common causes could be.

For info, it's a conventional boiler (not combi). It's a fairly old
Glo Worm system and it's a normal (not digital) mechanical clock that
it operates from.