Well, after nearly five hours, I've downloaded the latest maps and installed them in the satnav. Note in the download window was understating when it said "this could take a while" - it was several minutes before anything even appeared in the progress bar! And that was after taking the precaution of an ethernet connection between Netgear and MBP.
Annoying thing is you can only download maps for the whole of Europe.
Then it tells you your device does not have enough memory for all of them and offers you the bit you wanted in the first place (in our case, obviously, the UK).
Interesting that going to System Prefs after plugging in the cable to the MBP it had already switched over from Airport. Macs are just too darn clever!