I've managed to get the Peak District and Brecon Beacons maps to work. Previously, I had used Fast Image Unlocker to unlock them. It didnt report any problems so I assumed it had worked OK. I have now tried Garmin Unlocker Alternativev 5.71, which appears to have done the trick and these maps are now visible. Only problem now is that the Lake District map is already unlocked and GUA doesnt like that.

Nearly there!