With a bit of trial and error I cautiously managed to get the clip off and found that these things had seized, not sure what they are called though. I cleaned a load of excess gunk off and re-oiled them and everything now works fine.
Just need to sort out the springy contacts for the lights as they are not working consistently.
Thanks for all the advice guys, I wouldn't have known what to do if it wasn't for the help and advice on here, I'm really pleased I've got my loco back.
Steve.