bought a 1.6 AEE 8v octavia 1999 reg in October 2005 for

As for the question, no idea off-hand, but when I needed a sump for a Citroen ZX (porosity causing oil loss), it was under

I'm sure I read on here somewhere that if the threads go on your sump, you can drill them out completely, and then fit a sort of self-tapping adaptor that means you kind of end up with new thread.

If that's any good...?

Sounds like a Helicoil fitment to me. Not sure they really work on sumps due to lack of thickness of metal (and since you need to take the sump off to be sure of not winding up with swarf in the sump pan, not going to save much wrt a new sump at the price I quoted anyway).