I've got a 2005 Ford Fiesta Flame and it's got something wrong with the central locking. It's only started happening in the last few weeks, I've had the car since Feb. So here's the issue.

I can lock the car and it'll be fine then the alarm will go off for about 30-60 seconds then go off on its own. After this the car won't lock, it'll lock then immediately unlock. The door open light will be on during this time. I can not touch the car and after a few hours try to lock it and it'll lock fine until the next time the alarm goes off (Alarm goes off 2-3 times a week). Until today I was cleaning it and once I'd finished I tried to lock it and it wouldn't lock again and still won't lock. The door open light is on but nothing seems open. I thought it was a door sensor but it will sort itself out without anyone touching it so I'm not sure now. Any ideas?

This problem is caused by a faulty microswitch inside the door lock. The faulty microswitch causes the alarm and door open light to be activated. 

Replacing the door lock mechanism should solve the problem.

How can I find which door? 

Does the Fiesta instrument cluster show which door is open (while all doors are closed)?  If not the only way to determine which lock is at fault is by measuring the microswitch output using a multimeter.

Nope it doesn't. I've got an inclin which door it is as I brought it as a cat D and replaced 2 doors so I'm assuming it's either of them. Still have the damaged doors so going to take the old switches off them. 

There is a way to find which door is faulty without getting complicated tests done, within this forum I found a routine test procedure that identified which door was at fault.

1 ensure your window wipers are in the off position.

2 press and hold your heated rear window button

3 while still holding this button turn your ignition key to position 2 (2 clicks) leave the key in that position

4 release the heated window button, then quickly press the heated window button 8 times within 6 seconds.

5 you should now be in test mode, one at a time, preferably with an assistant, open each door you will hear a beep with each functional door, the faulty one will not create any noise.

taking the offending door panel off Is ok as long as you know what you are doing.