I don't like to be facetious in the face of what could turn out to be another disastrous episode, but the fields alongside the laboratories were the site of the Royal Egham Show at the end of August.
Of course there were no sheep, cattle or pigs, but there were guinea pigs and rabbits. Now the DEFRA site tells us that rodents are susceptible species (except for pets) and I assume the rabbits and cavies concerned were classed as pets so therefore could not have been infected but I wonder if any of them came from Pirbright?