Think you'll find fusing and it's use on bikes has moved on a lot since those days mate.
Keep on upping the fuse rating (as suggested above) isn't the way to go either. Far better to find the fault that's causing the issues, and a Thorspark unit (assuming it's not faulty) isn't going to blow a 5A fuse by itself.
Yes, I agree Rex. On the face of it it isn't logical that it would blow the fuse when running if it
didn't blow when all loads were switched on with the engine off.
So I think there might be a problem over and above the inadequate fuse rating. (10A is usually good on 12v dynamo bikes, I go with that generally.)
You could check whether D and F are correct at the dynamo (even if they seem to be plugged into the right holes if you've got a plastic / bakelite cover on the end) by whipping the cover off and looking to see: should have D coming off one brush, possibly by way of a terminal blade or post on the brush-plate, and F should be similarly connected to one of the wires coming out of the guts of the dynamo through a hole in the plate. They should obvs go to D and F at the regulator.
However, reversing D and F by mistake won't normally blow a fuse - you just won't get a charge. You'd probably get uncontrolled charging though if there were any connection between D and F on the dynamo.
What does your
ammeter tell you though? It should be your friend in this.