Just did a bit more exact measuring (find this a bit tricky!) and Timmy is about 45/46MM and Betie is about 49/50MM.
To be honest I made this thread as I was a bit worried I may be over-feeding and they may have been gaining weight too fast!
Thanks for the replys, as long as people don't think their weight gain is too fast, think they'll be fine, as they seem perfectly healthy/active/eating well.
They were from a lady that uses this forum and I'm sure they will have been healthy when I got them, just want to make sure they stay that way!