Eloathan, I do not think this attack has anything to do with Porton Down, but this is the seed of doubt that Putin and his mates would like to sow in our minds.
I agree M0nica
There has been a lot of speculation about this, the logic of some of which eludes me.
The first attack was in Salisbury because the target, a Russian double agent, lived in that area.
The second incident is in the same area, one would presume logically, because that is where the remains of the first attack have been disposed of.
Whether the first attack was by Russian or by a rogue agent/terrorist who has (frighteningly) managed to get hold of a supply of Novichok, is yet to be established.