I recently tried to replace the above mentioned central locking case for a customer. On my first visit I installed an emergency over night locking case and brought home their old lock.
The problem with it was easy to see once I removed the cover. A very small spring had become dis-lodged and was actually restricting full movement of the bolt. This spring is approx 5mm long and only about 3mm in diameter. It was actually standing in an upright position inside the longitudinal slot in the bolt profile. There is another similar spring which is even shorter which lays flat and fits in a vertical position behind the bolt plate. This spring works against the cam when the key is turned and forces the plate behind the bolt plate back down to its normal position once the bolt has fully extended.
The first (dislodged) spring mentioned appears to be there to help the bolt to fully retract when unlocking. As without it installed the bolt protrudes by a couple of mm.
I wondered if anyone had come across one of these locks and knows where this tiny spring fits?