Does the lens lock firmly into position or do you feel some wiggle when it is locked? There should be none. There is definitely excessive wear on the sliding rail of your camera, maybe the result of someone applying undue force when trying to push the lens into the body with the shutter uncocked ... most probably cannot be repaired in a classic fashion, as the rail appears to be an integral part of the body.
As a makeshift solution, AgX's suggestion could work, there is tiny bit of room left with the lens fully seated in the body, but as the parts involved are moving and you have to rotate the barrel to lock it in the extended position there is some risk that anything you attach there could loosen sooner or later and become entangled in the sliding mechanism or the shutter actuating mechanism on the left and lock things up.
You could also try if it is possible to block the light leak from the outside, with the help of a simple rubber ring around the lens barrel where it enters the body. Best solution would be to look for a replacement body in good shape.
Thanks. The lens locks firmly into extended position, no ‘wiggle’ at all.
The leak appears (looking through the camera from behind) in the 4 o clock position, where the mechanism to twist and lock is, so not a good place to address from the inside.
I’ll try a hair band or something similar on the outside of the barrel. There is significant play in the plastic DOF collar on the lens itself, though I can’t see that would have any effect.
I’d be very loathe to part with A$300+ for another with the meter on top, purely b/c there seems little guarantee i wouldn’t be trading one problem for another.