Update!
I have located the problem.
Turns out is just have been an underlying one when I purchased as the trim had been neatly cut just to expose the side in question.
Popped it off, had a little bit of a fiddle around. The left 'hook' is very weak and has a lot of give it it when locked, once locked it. A little push up on the roof and it pops out.
Have attempted tightening but irrelevant as it's the actual hook that's gone.
So really only option I see is to get a new lock in mechanism. Looks fairly straightforward to change.
Which leads me onto the next question of where is best to get one from....