My 1997 (P reg) ZX has a similar problem, thanks to a child jamming a
'foreign body' into the driver's lock, and someone trying to force the lock
of the rear door.  I had intended to ask around in local repair shops when
I feel wealthy enough, so I too would like any better suggestions.

You can get 'universal' remote-locking kits, which would also overcome the
jammed key-hole problem, and might be cheaper than replacing most or all of
the lock barrels.  There's one in the Maplin catalogue for GBP50 that works
with an existing 'central locking' system.  If your car doesn't have central
locking, you can get kits for that too.