Just a thought ,( I know it's a different model but it had the same symptoms)
Yesterday I had a customers mk4 polos with a similar problem ( it is an S reg with the later laser cut keys )
what had happened in this case : the mechanism in the drivers door key barrel had broken which in turn stopped the inner paddle being turned by the key ( stuck in the halfway position ) thus confusing the micro switches for the central locking . I replaced the broken items .. problem solved.!
The problem with the mk4 I have been told that you cannot get the separate parts of the mechanism , you have to order the whole barrel etc by key number from the factory , though not sure how true this is .
Maybe a good idea to check this before ordering a new latch assembly ( probably is the latch and I know it's a long shot but worth a look )
