Ok, So I got a relay and tried to wire it up as per your diagram, however the relay is slightly different, i.e. the 85 and 86 are on opposite sides and the 87 and 87a pins are both labelled 87. no big deal from the look of it, should do exactly the same thing. However, regardless of whether I connect the door lock trigger to 85 and the power to the 86 or vice versa when I hit the fob button to send the signal, nothing happens.
Tested the remote system (i.e. lock/unlock triggers) with a test light and pushing the button completes the circuit, looks like that's working fine.

I'm probably being a numpty and missing something, so I got my mate (who used to do sound/alarm system installs for a living) to have a poke at it for me, he came up with the same thing.

The only thing I can think is that the Relay is the wrong type (Normally Open vs Normally Closed?), or is cactus.

Am I missing something?