Central locking packed up today :(

[S8yot]

No previous problems at all. The pump runs (for longer than usual) but fails to lock or unlock the doors. Makes no difference using key rather than remote. Alarm sets and interior lights go on & off correctly when the drivers door is locked/unlocked on the key.
Having had a root around in the boot and removing the air lines from the pump there doesn't seem to be any discernable pressure on the inlet/outlets on the pump when it runs. *How much pressure should it produce? Assuming it is the pump, what's the best option - secondhand or is repair feasible? *