Ford Focus Ghia x reg - Central Locking not working?

Hi,
I have the above, we do not have a remote key as this was not with the car when we bought it, but we do have the blue chip key which we have been using but now every so often it wont open the doors, to get them open i have to turn the key from left to right several times, this can take quite a while & is quite frustrating, any ideas, would it be the fuse??

Regards
Tim