I thought the fuse should be in the brown/blue because that's where it is on all the Lucas wiring diagrams I have seen, but clearly one in both is safest. Also, I should have twigged the cable rating is lot less than the fuse blow so of course, it makes sense that unless its a dead short its possible to cook and melt the cable without blowing the fuse. Modern wiring has multi fuse boxes for each circuit and I can see the sense in these now - Doh!
Re the fuses, I always get my Blade and Tube fuses from Vehicle Wiring Products and they confirmed the rating on the fuse is blow so I'm confident I have the correct ones in there. Indeed, when buggering about attaching the battery to my Royal Star last week a slight touch in the wrong place and the 15A cont-30A blow fuse popped immediately.
So many thanks Stuart for the benefit of your knowledge and thanks to Jimmy for having the problem in the first place!