I read that the woman affected lived in a flat near the Italian restaurant which the Skripals visited. That seems to mean that the decontamination process may have been lacking. The restaurant is still closed and sealed off though, so I'm not sure how a nearby flat would be affected.
This is all very mysterious but the overwhelming feeling must be sympathy for those involved, especially the latest couple, who seem to have no connection with Russia, Porton Down or anything else, other than where they live.
I live near Salisbury and have visited many times, including when large parts of the centre was closed off and business were feeling the pinch then. There were noticeably fewer people about, though it was just beginning to recover. Such a pity, as I said, for all those affected.
Having said all that, and as someone who has always lived near the area, I was always aware that Porton Down was a mysterious place ... it used to be the Common Cold Research Centre when I was a teenager, where people could have a 'break' for free as long as they were willing to accept being infected with a common cold virus. Seemed odd to me and still does. From memory, I think participants had to sign a non-disclosure form ... I may be wrong. If anyone knows better, please put me right.