I had this happen on a couple of occasions, and after fuse was replaced, all was well for a few months, then it happened again. A knowledgeable friend told me to examine the blown fuse, if the element has broken in the centre then it's a short circuit, but if the element has parted company with one of the end caps, then it has been struggling to pull current. This was the problem in my case, and I was told to examine the white multi plug located behind the fuse holder board, and lo! when pulling the plug apart all the blade terminals were coated with 40 years worth of green gunge. 30 minutes work with a set of small jewellers files cured it, and then the whole ensemble was wrapped in insulating tape. DON'T forget to disconnect the battery earth lead before doing this. Be lucky, Michael.