So you did purchase the update for 50 quid then i assume. The fact that you get the 'error installing update' when trying to update the SD card with GE indicates to me that either (i) There may be a problem with the card rather than the device; or (ii) The GarminDevice.xml on the card has some problem causing GE to throw that error. If it's the latter then perhaps the XML is reflecting some problem with the device itself.

If you've indeed forked over the money you should be able to get some assistance from Garmin UK. I think the map runs directly from the card in your device, i.e. the head unit isn't updated with a new map from the card. So if it runs directly from the card and GE hasn't updated the card i would think that it's not unexpected that the system setup reports the maps are 'missing'.

The immediate action for any device not performing normally is to do a hard (or factory) reset. As it seems the card itself isn't being fully recognized then use another card, essential if you'd previously properly reformated the present card. Try doing both reset and update using another card with a fresh XML from the head unit before re-connecting to GE.