Potterton boiler blowing it's fuse?

Potterton boiler blowing it's fuse

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I have a Potterton wall mounted boiler that decided to blow a fuse the other day shortly after coming on. There was a smell of burning electrics, although apart from the blown fuse I can't see any obvious damage. I tried replacing the fuse and turned the boiler on again immediately it blow again

Any help would be much appreciated.