Nugget is going to see the vet tomorrow afternoon to get the lump looked at - it's underneath to one side a bit. He seems fine in himself and was out and about in the cage last night being active and running on his saucer and looking quite perky. Here's a very short video of him in the playpen the other day. He ran around the playpen for ages sniffing, exploring and running through a long tube, and using the saucer and having a wash - seemed happy being in the playpen - but then started scratching and chewing at the corners which is when I put him back in his cage.
So here he is. Nice to have a close up video of him. I think the lump is causing him some practical difficulties climbing onto things but that may be his age as well. He is very active otherwise.