Oh, after over a year of ownership I have realised that with our keys in a metal bowl by the door it is possible to open the car using the door button!
I had assumed the thickness of the wall and the metal bowl would have stopped this from being the case.
We have now bought a faraday cage box for the keys to go in.
I did check and it wasn't possible to drive off with the keys in the house, only to unlock and lock.
But I suppose that means the relay trick might have worked easily in our case.
Oooops!