This evening after work I approached my car and pressed the 'unlock' key, but notihng happened. Pressed it quite a few times. I tried the 'lock' button as well, and nothing happened. Once I'd unlocked the car manually, I tried the buttons again and they worked fine. Is this a fault or is it the immobiliser coming on for some reason, and thus you can only then unlock it and disable the immobiliser by using the key first?

Try replacing the battery in the FOB, could be running low. Eliminate that before you look into it more.

Mine did the same, ended uo changing the door lock- RCL receiver is built into it if IRC

Nah, it's not the battery, as it works perfectly once the car is unlocked manually, and still works from 30 odd yards away most of the time. I guess it may be the RCL receiver then :/ Guess I will live with it until it gets worse and see how much it costs. Thanks for the advice.

Could the fob be being pressed in your pocket? If the buttons are pressed too many times out of range of the car , the car will not recognise the code when you try and unlock it (maybe other keys/change etc in your pocket operating the buttons?) The solution is simply unlock and lock the car with the key , it re-finds the key code this way and it works again - at least that is how I remember it!