I think what you are dealing with is the difference in fuse ratings between Lucas and standard american fuses. From the above post, you can see the amperage ratings of american fuses are about half the number of Lucas. That apparently is because Lucas is measured differently. Therefore if you put an American fuse in there with the same number of the Lucas that came out you could overload the circuit and burn some wires. Also, in my TR7 I noticed that the Lucas fuses are longer, so the American fuses don't fit as well.
Then again, maybe you just need a good injection of Lucas smoke. I think Randall has the Lucas smoke injector kit. ; )