So, Porton Down's CE claims not only that they have not identified the source of the 'nerve agent' but that
'it is not our job to say where it was manufactured'.
https://news.sky.com/story/porton-down-experts-unable-to-identify-precise-source-of-novichok-that-poisoned-spy-11315387
In that case, where does this leave the Foreign Secretary, who responded thus in a recent interview:
'DW: You argue that the source of this nerve agent, Novichok, is Russia. How did you manage to find it out so quickly? Does Britain possess samples of it?
BJ: Let me be clear with you … When I look at the evidence, I mean the people from Porton Down, the laboratory …
DW: So they have the samples …
BJ: They do. And they were absolutely categorical and I asked the guy myself, I said, 'Are you sure?' And he said there's no doubt.'
http://www.dw.com/en/boris-johnson-russias-position-in-skripal-case-is-increasingly-bizarre/a-43043873
I expect, being the principled man that he is, he'll be stepping down soon. Or that if he doesn't the Pry Mincer, being the principled woman that she is, will be asking him to step down. After all, I'm sure she wouldn't want a liar in her cabinet.