Turbo timer.

[Niall]

Right but if the TT runs in line with the ignition, the BSI etc will still think the key's in the ignition surely?

Correct but your central locking wont work until the BSI has gone from running to idle and neither will your alarm. And it would be pointless getting out of the car, going in doors then coming out a while later and locking it.
you could make a simple extra circuit so when the timer runs out, it locks the doors but i never have trusted automatic central locking. Had it on a cheap alarm on my 106 and i ended up locking my keys in the car several times