Car Door Unlocking Problem?

Hi,

I have a Ford Focus 2005 2.0 TDCI and a while ago one of the rear car doors wouldn't unlock with the key, remotely or using the key in the lock. I also couldn't physically prise the lock open from the inside, it just doesn't budge. I did, however, find that the only way to open the door was to start the engine and then lock and unlock the drivers door (that controls all the doors) but as I say, this only works if the engine is running.

My proplem also moved on to the drivers door once, however this began working again and has been ok since. But now all other 3 doors, except the drivers door, have the same issue. I now try to leave the car unlocked whenever I can in case I can't get in!

I've read posts about fuses and grease, but if the fuse had gone or the lock needed greasing then I wouldn't be able to open it by locking and unlocking the drivers door with the engine running, it surely wouldn't open at all. I've done things like swap the fuses over just to test but it made no difference.

A garage once took a look and got it working for a day but then it stopped working again, but I don't know what they tried. If anyone can point me in the right direction and suggest anything that would be great. Thanks.