If (as you say) your ZR has the squarish Lucas fob/5as security system, it will have been built prior to 30th June 2003 (VIN number 732813 or lower).
Up until 15th July 2002, remote central locking was not fitted as standard to the base model of ZR, so if your car is a base model 105 and was built before that date (VIN number lower than 671786), it will not be fitted with remote central locking - the remote plip/fob will only arm/disarm the security system/immobiliser. In this case you will have a key lock in the door on the passenger side, which will not be fitted if the car has remote central locking. It is sometimes the case that if the battery in the fob is becoming depleted it doesn't transmit the signal clearly enough for the 5as to repond, but that normally results in neither the CL nor the alarm/immobiliser responding.
If it was built after 15th July 2002 and has a higher VIN than 671786, then it should be fitted with remote central locking, and I would think perhaps the most likely reason for it not working is probably failure/damage to the microswitch on the drivers door lock which tells the 5as security unit whether the door is locked or unlocked.