Hmmm, even weirder, turning off security on the primary wireless network makes it work - Obviously not a sensible thing to leave running though
So, deffo not the hardware, just some bizarre hidden setting or NVRAM corruption.
Seems to be running happily with the virtual network, so I'll leave it until I get time to flash one of my spares with the same version, then I can quickly swap it in/out and see what happens.
Also, if anyone knows of a wireless router with per-MAC bandwidth throttling, I'd like to hear about it
Al.