For close to 8 months II have had no problem with this alarm. When  there is no dash board light, I can open and close the reportedly faulty door 100 times and it will work perfectly every time suggesting it is not a mechanical or an electrical fault with door switch. if the car stands for a while the fault clears on its own..  Does anyone know if I buy a cheap OBD2 reader will I be able to reset this fault. in case it is causing the problem. Thanks in advance.

If you’re planning on staying with Ford for a while, one of these modified Elm327 readers coupled with Forscan on a laptop is the best code reader you'll get without spending hundreds on Ford IDS.

7 hours ago, Stewpot said:

For close to 8 months II have had no problem with this alarm. When  there is no dash board light, I can open and close the reportedly faulty door 100 times and it will work perfectly every time suggesting it is not a mechanical or an electrical fault with door switch. if the car stands for a while the fault clears on its own..  Does anyone know if I buy a cheap OBD2 reader will I be able to reset this fault. in case it is causing the problem. Thanks in advance.

This is a known problem of the Focus MK2/MK2.5. The problem is caused by the microswitch inside the lock mechanism. During time the grease/lubricant inside the lock becomes hard/sticky and intermittently prevents the microswitch from switching correctly. The switch will simply be stuck in the switched position before it comes loose again. This problem can be very intermittent. However it usually gets worse. 

This problem can be solved by disassembling, cleaning and relubricating the lock mechanism. For lubrication a very thin temperature resistant grease is required. This is however a pretty labour intensive job. It is much easier to just replace the lock mechanism. 

Once again the alarm has reset without intervention. Now I wait to see if it will return. What loony thought up this crazy system. When I widen my search, I find this happens to all makes of cars , yet no one has managed to solve it. The problem is bigger than a micro switch.

On 2000s VAG cars it's down to poor solder joints, same symptoms, but not the same cause.  Used lock mechs are cheap and pretty easy to replace if you have a few tools.