Time Machine back up to Time Capsule fails
Usually, I wouldn't post an answer like this  (rather, just delete the question) as my solution to this issue is so untechnical; however, I feel that it might just save somebody a head of hair.

It turns out that the Ethernet capable connecting the Time Capsule to the network switch was damaged. I never checked it initially because the Time Capsule was appearing on the network and my MacBook Air would back up to it fine, but all I can assume is that my MacBook Pro but more stress on the cable. Long story short, I've put a brand new Ethernet cable in and it worked first time, flawlessly.

Now, before I did so I had followed this tutorial to keep using the same machine after a change to the computer, so I don't know if that did have any effect or not.

I suppose the moral of the story here is that networking is a pain and you shouldn't overlook anything, even if it seems to be working perfectly!