To be fair, the unit was some years old, and had given stirling service up to that point through two daughters (now all growed up and gorn) and the missus, so I can't really complain that the switches were not of good original quality, and fit for purpose. As to fusing, the plugtop fuse was certainly never going to blow as a result of the primary failure, because at that point, as far as the fuse was concerned, its load wasn't doing anything wrong. In fact as the thing was running - you'll remember that I said in the OP that it sounded slow and laboured - as a result of a carbonised leakage path across the switch, it will have been drawing a less-than-normal amount of current through the fuse at that time. Left to carry on, I guess that enough insulation might have gone west to finally create a fuse-blowing short, but by that time, I feel that it would have been well alight ...
Arfa