If it is not catching, then either the new pin you have chosen is not quite long enough or the hinge pivots have become partially displaced and the mechanism has move to one side of the tray rather than being central.

 

Although the mechanism looks complex, in realiity it is really a bit like an over centre rocking horse. the pin rides in the slot till the weight of the lid makes it drop into the little vertical slot. When you push the lid to open, the pin rises from the slot and the spring moves it into the diverting upper channel and the lid stays open

 

 

Thanks Phil I'll keep trying different things 

Well I've knackered it. I tried to melt a screw through the plastic bit on end of folding lid but melted all the plastic and the screw keeps getting jammed so I just took the spring off and out it all back together in the car 

I'll just have to manually open and **** it now ! What a drag lol

solution available under my post(in a while):

 

reasonable solution updated.