I checked it this morning and the kink was gone!!

There's still a teeeeny tiny lump, but nothing like there was last night. He wasn't in any noticeable pain last night, even after me and OH felt it (he was just uncomfortable), he was whipping it around the way he normally does with no problems and is still doing so this morning.
As they have the vets a week Monday anyway, I think I'll leave it til then. It doesn't seem to be hurting him, he's still happily playing and is currently sunning himself on the windowsill.