Thanks all. I've worked it out because of my other thread.

The AVG firewall had blocked other stuff and I wondered if it could be blocking this too. (Admittedly I'd updated NAS firmware by this time too.)

"File and Printer Sharing" was disabled in the AVG firewall settings. I enabled it and hey presto the device appeared in the network list and I could click through to my shares and map them as drives.

It looks like AVG got corrupted somewhere and not only caused my other problem but this one too. Phew!