The red LED on the multi-function keypad constantly flashes rapidly whether the engine is running or the ignition is on or off and disabling it from use. Having read some other articles on this site it seems the problem may be a damaged membrane, is this correct?
Can replacement keypads be purchased, where and at what cost?

 

Yep, sounds like a damaged membrane, one of the digits will be in constant contact simulating a repeated false PIN attempt.
I'm not sure if the dealers are aware of the part number for the membrane on its own, but it should be SACC 3463-CVR. This can be used where a simple membrane change will cure the problem.
If the membrane on it's own is hard to come by or the keypad proves to be more seriously damaged then the part number for this as a whole is SACC 3463 and should be arround £19 from a dealer.

Once the keypad/membrane is replaced, the system will need to be left for 30 minutes before the new keypad can be used as the alarm unit will have "locked-up" as a security measure and ignore any keypad enteries due to the incorrect PIN attempts.

SS