While I agrre with the assessment of both Putin and Tories, I don't agree with the assessment of assorted governments. As we saw in the case of liquid on airplanes, governments can be sent into stampede by rumours and then have a hard time backing down.

I have been folllowing Craig Murray on his quest to read exactly what Porton Down is saying, and a recent piece is this part of the judgement on allowing OPCW to take samples from the Skripals:

The Evidence

16. The evidence in support of the application is contained within the applications themselves (in particular the Forms COP 3) and the witness statements.
    
17. I consider the following to be the relevant parts of the evidence. I shall identify the witnesses only by their role and shall summarise the essential elements of their evidence.
    

i) CC: Porton Down Chemical and Biological Analyst
Blood samples from Sergei Skripal and Yulia Skripal were analysed and the findings indicated exposure to a nerve agent or related compound. The samples tested positive for the presence of a Novichok class nerve agent OR CLOSELY RELATED AGENT.

Murray's emphasis, and as he points out Novichok class nerve agents can be manufactured by other parties because Iran did so in 2016 (in full cooperation with OPCW). Until OPCW has taken their samples and done their testing, that is were the chemical evidence appears to stand right now.