Well, I think, touch wood, it has finally loaded. Sequence of events:
Started car, safely removed old card. Inserted new one, maps not found error on the home screen
Got to destination, did shopping, started car for journey back. R-Link starts and says Loading map, one moment please, gets stuck and reboots R-Link
This happens several times, it's in a loop. Arrived at a friends house, car off
Get in car to go home after about half an hour, start up and, ta da, map is installed! It has lost recent destinations and home for navigation but reset that once back home.
Will see what happens! Not planning on using the car for a few days now so hopefully it'll be OK.