Update 2. Tested PII BX machine with Knoppix 4.0.2 CD (I should have realised I'd not tested this earlier). Odd results. The NIC fails to function exactly the same way as later kernels but the io base address shown by the 3c59x module and ifconfig is 0x6800, the normal expected address. 4.0.2 is kernel 2.6.12. Also tried Kanotix 04-2005 and Helix 1.7. Both appear to use the same 2.6.14-kanotix-9 kernel. NIC fails to functions as before and the address shown is the strange 32bit one. I tried Debian kernels 2.6.15-1-486, in case it was a 686 quirk, upto the latest 2.6.17-1-686. All fail the same way as previously described. I read that someone was having problems with drivers compiled using gcc 4 (I think it was this forum but can't find it again) so I recompiled the 2.6.17 kernel using gcc 3.3.6. Exact same results.

I'm beginning to think that something broke around 2.6.12 that stopped this machine working. After that something else caused the strange 32bit address thing but it isn't enough to stop my other machines from working. That is, the two things may not be necessarily connected.

I've taken this much further that I originally expected. I can't do much more unless I delve into the source code. 'Spose it's one way to get to know Linux better...
Still on-one else with something similar?