central locking doesn't unlock

I've an intermittant problem on my 328 saloon. In a morning when engine is cold, central locking works fine and locks/unlocks all doors. When I've been out in the car and the engine has warmed up, everything will lock, but the doors will not unlock - the only thing that opens is the boot.

The only way I can gain entry to the car is physically unlock the drivers door with the key but cannot open the passenger or rear doors and fuel cap, as they remain dead-locked. Only occasionally will they unlock.

Fueses all ok, are there any relays that I can check or has anyone any other ideas?

Its an E46 model, year 2000

Thanks