Hi Mike
I had similar problems with mine when i first got it , the headlight fuse had been replaced with a piece of wire , i replaced it with a proper fuse and it lasted one ride before it blew, this happened a few times and i started to understand that my bike has an issue with the connections or something in the terminal block. I bypassed the fuse holder and eventually realised the problem was between the fuse holder and the block , between the front and back of the bakelite panel.. so hard to find and fix , i have bypassed that connection , its not as neat as the original but its still working fine now .
cheers
Ric