The rubber surround on the old one was falling off so it was replaced under warranty. Turn-round was 4 days. Impressive.
I put the Sd card in and was rewarded, on switching on, with the friendly message "Cannot unlock maps".
I phone Garmin who immediately sent me an email with my 25 digit unlock code, but for which I would have to retrieve the CD that came with the machine (wherever it is) and load it into a PC, which we don't have (all MAC here). Dez had a PC when I bought the machine and we loaded the maps on from that.
When I told him that, he said "OK, there's another way of doing this" and he then emailed me an attachement names "gmapsupp.unl" which I dragged & dropped into the Garmin folder on the card.
As if by magic, my blood pressure returned to something close to normal and the Garmin works.