I took peter n's advice and gave the inside a closer inspection. I did find a piece of ripped film in the take up spool and its internal spring was distorted so I sorted this out. I then tried the rewind again and decided to apply a bit more pressure praying it wouldn't make matters worse. It took only a little more and it suddenly freed up and the shutter began firing again. This can't be normal in a high precision mechanical device can it?
Seems Ok now so I'll put some film through in the next couple of days and hope it turns out fine and then forget the whole thing happened.
My MP is no longer jammed and back in the game.😀