Just out of interest, having bought a house here recently, if the diagnostics report showed that there were no termites and they were then discovered shortly after moving into the property - what cOuld the purchaser do?

Maybe that is correct but if the Artisan has the proper equipment like sensitive listening devices then they could hear any boring bugs having a scratch of their heads wondering what to eat next.

Someones not done their expert job properly so as Suzanne said.

Sometimes there is nothing to be done. If the house and surrounds have been thoroughly surveyed and diagnosed there is no actual reason why a colony did not expand in just a few weeks and the first scout termites be tunnelling in, which is already an infestation. We are in an area of fairly high risk and have heard just how fast the little buggers can turn up and keep the extermination companies in business.

It could be considered a vice cache - have a look here http://droit-finances.commentcamarche.net/faq/2815-termites-et-insectes-xylophages-vice-cache

I think it would depend upon whether or not the experts were able to have access to all parts of the house. If not (because the roof timbers were clad underneath with a covering and they did not have access, for example) they may have a get out of jail free card...