Cheers for your answers guys! I have however bypassed the French BB problem. Forever! And no-one will ever know from the outside. You see, as the threads were worn out anyway, I was left with two options; hardsolder some solder into the BB Tube and rethread, or (more rigourously but certainly much stronger and safer) cut the BB tube, squeeze together, reweld and rethread. I opted for the latter. So now I can forget about all that French thread stuff forever. The bike has been in the family for almost 30 years, in my possession for 15 of those, and I was really hoping for at least another 30!

Next step was to stick in the BB, but this is where the problem now lies. I have looked at the D/A Octalink BBs, and my axle length choice is apparently limited to 109 or 118. 109 seems to fit fine if I stick in a square taper BB, but I don't know if Octalink 105 cranks fit on at the same distance from the side of the BB tube as the square taper. My usual LBS would be happy for me to try both sizes, only issue is they don't have the D/A BB in stock. And they would charge me 2x what I would pay on Chainreactioncycles for the same BB anyway, so I don't want to take advantage by trying it and saying 'thanks, but I'll buy elsewhere'...I like those guys too much for that. And I also don't see the point in spending the extra £££, as I could use this for some of the other parts I still need. So, I am hoping someone on this site can help me. Otherwise I will just have to buy both, try them, and see which one fits best, and then sell the other one on ebay or something.