When l press the button on my key fob the car unlocks then locks streight away

It sounds like the solenoid in the car (recieving sensor) could need replacing. You may find that the fob needs reprogramming?

Either way head off to Citroen to ask their opinion.

Good Luck.

well there is probably a falt in the electrics bit for now use the main key

Well as its a Citroen prehaps its telling you to get a decent car and locking you out. That way you get a new car lol

It is quite possible that this problem is from the cold, (has it been cold recently) Parts of the central locking can get a bit damp inside the door casing and when the temperature drops below freezing can freeze resulting in poor operation of the of the unlocking mechanism. A good squirt of WD40 should sort out the problem but you may have to take the door trim off first.

You bought a Citreon, what d'ya expect mate?

most proably the solenoids, get them replaced son but best just using key for now.