Jon,
see my post of Dec 4th, entitled "Central locking for £20!"
Would be useful to have pictures and wot-not, as well as being
slightly better written.
As a side note, in order to interface the central locking to an
alarm/imobalizer (be it after-market or Rover-fitted), you also need
to purchase the interface kit from Maplin (only about £15 as I
remember).
The output from the alarm is a "grounded when armed" type output (on
99% of alarms, anyway). The interface box looks for this being
switched and then gives a timed output to lock or unlock the doors.
It's pretty easy to wire up, although the circuit diagrams in the
manual (I call it manual mockingly) for the interface are a bit scary.
Anyway, if I can be of any help in your fitting of doo-dads, just
e-mail me and I'll be glad to assist!

Dan