Ok, this seems to have resolved itself for now
Tried signing-out of X-Box Live and back in again, and this time I got a message when I tried to join the Realm along the lines of 'Your Realm will be updated soon. You will need to be updated as well' Clicking continue gets me stuck in the same loop as before.
OK, it would appear that it is just myself, the Realm 'owner' that is experiencing this problem. My son, and I believe some of his mates, can play on the Realm without any issues.
Ok, this issue is not resolved...I can no longer access my own Realm as its owner, still stuck on ‘Building Terrain’. We pay good money for this service every month, and a lot of the time one or more of my lads Minecraft buddies are unable to play!