central locking sticking

Just wondering if anyone else has had problems with the remote central locking on the XS models? The locks on three of my doors are now so stiff that they are no longer activated by the central locking. It was worse still over the recent cold period. Q: Is there anything obvious I should be lubricating? I recall how difficult it is to remove the door linings without also knackering the plastic clips that hold them in place! Ta. James