When I eventually found the right screwdriver (long story, my son had put them back somewhere other than where I normally keep them), I opened the case and the network connector lead had come off the Q68 circuit board. I'd tried Martin's suggestion of shaking it to try to hear if something was loose, without hearing anything, so maybe it was half off and only after removing the case top did it come completely off.
Good job Derek mentioned the alignment of the black paint on the network leads connector as it isn't immediately obvious which end of the expansion connector is which, in case I put it back on the wrong end - the twist of the cable implied it went at the end nearest the back of the case, so thanks for that suggestion, Derek.
All working fine now.
Martyn - glad to say that v3.05 of ndq68_dvr_v305 works with the combination of equipment listed below, will try the other software version later. I had FSERVE going on all three machines, all accessed each others' drives OK. I'll see what other equipment I can put on the network tonight, maybe try the second QL without its QIMSI-Gold and the backup BBQL if I can find it in the attic. None of my PCs have a serial port to try adding Sernet to the mix, though if I can find a suitable RS232 cable I might try the spare BBQL via Sernet to the Q68 just for (slow) fun. Then another time try Martin's IPnet driver to introduce the Q68 to the PCs
(I can hear you all gnashing your teeth in worry...)
So then I'll probably have the second or third largest QL network outside of the Martyn Hill household and possibly Derek's setup.
NET 1 : JS QL/Gold Card/QIMSI (SD storage only)
NET 2 : Q68
NET 3 : Aurora JM/Super Gold Card/Qubide/ROMdisq (Q-Branch Minis-QL system)
(tried Aurora with SMSQ/E too, also works OK)
Interesting that it's all seemingly quite reliable with a mix of QDOS and SMSQ/E in the chain.
Even silly things like DIR n1_n2_win1_ seem to work, albeit slowly.
Long time since I played with the QL network like this! Will see if I can find the rudimentary network messaging software I wrote to use to use at Quanta workshops in the 1980s, if the floppy disk still works.