Thanks guys - all your advice is much appreciated. I think I will hand the camera and lens over to a local repair shop. John Hermanson's diagnosis appears spot on. There's black, sticky goo on the bottom of the back and, although I can't see any evidence from the negs, I suspected that the seals might have gone when I spotted the crud coming off onto my fingers.
As this is my little collector's piece, I think I will treat it to some professional TLC as I want to have this camera for a long time. As I said in my earlier note, I am a tad cack-handed and I really wouldn't trust myself with a precision instrument - especially as it was a Christmas present from my wife.
Regards, PJ.