Central locking on astra Mk5 ?

can anyone help me out please my central locking stopped working only the boot locks when I take key out of egnition everything turns off as it should the only way I can lock the car is to disconnect the battery then reconnect it then the doors will lock and unlock but just the once the back to not locking again had it on diognostic and no faults showing could it be the module if so what and where am I looking for ? Any help would be appreciated thanks a lot Dave