Took the offending vehicle + spare key + code card to the dealership in Lee just outside Lewisham.
Was told it would be done whilst I wait and would take about 1/2 hour. In the event, took 1 hour but was fixed.
Delay was simple re-coding of keys did not work and they had to "re-boot the whole system and re-code the keys again, hence the delay". Bill was £40.50 + VAT = £47.58, which was what I was quoted for simple re-code of the keys, so no extra charge for re-booting the whole system.
A word about the dealer, they were friendly and helpful and free coffee was supplied during my wait. A good experience, bearing in mind the criticism of dealers in general and particularly Citroen dealers one reads about on various sites.
Chris Brayley
November 2008