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....