Followed Marcus S's advice, drilling out bolts etc.
Then found fault was as described by Jon. with one slight difference, the metal cross section protruding from the rear of the lock had rounded at the end so wasn't locating fully into the plastic rod.
As per his description though, tied a small cable tie around the cross end on the plastic rod, this forced the metal part fully into the plastic and oh joy, all working properly.
Many thanks to the two gents mentioned above, you've saved me two or three big ones !!
Alan M
March 2012