Well spotted Jibber... no bright marks on the gears Dave, it's a reflection off the flash.....
When I took all the screws out of the casing, I gave it a tap with my mallet to free it off & the there was a loud clang as the spring let go from wherever Bruce had it wedged & it hit the inside of the case.... it also whacked my fingers as I eased the case off over the dowels....
Can't see anything else wrong, nothing fell out so he didn't have anything in there holding the spring.
I'll fit a stopper & re-assemble it, see how it is...

The future is not what it used to be....