The Foreign Office is now saying:
It is our assessment that Russia was responsible for this brazen and reckless act and, as the international community agrees, there is no other plausible explanation.
As the Prime Minister has set out in a number of statements to the Commons since March 12, this includes our knowledge that within the last decade, Russia has investigated ways of delivering nerve agents probably for assassination and as part of this programme has produced and stockpiled small quantities of Novichoks; Russia’s record of conducting state-sponsored assassinations; and our assessment that Russia views former intelligence officers as targets. 
This is really very thin. It wouldn’t stand up in a criminal court. That the act was “brazen and reckless” is just there to add some colour – a barrister’s trick; he hopes to sway the jury just by stressing the enormity of the crime. But in reality this has no bearing on who did it.
The “international community” (our military allies) agrees because – as for example Merkel explained  they have been presented with some convincing evidence by the British. Now, apparently, the fact that they agree is itself proof of the truth of the claims. This is a circular argument.
It would be interesting to know the basis of the claims about Russia stockpiling small quantities of “Novichoks”. At any event Porton Down has not said that they don’t. Former British Ambassador Craig Murray suggests that the form of words used by the representative from Porton Down on Sky News today is as good as an admission that they have produced it.  If this is so – we can ask; if Britain can why not Russia? The claim about producing the substance “probably for assassination” probably (!) relies on claims made by a defector.
Russia’s “record of conducting state-sponsored assassinations”. Such as? Again this is looking rather like circular reasoning. And “our assumption”. This is why this case wouldn’t stand up to the rigours of a proper criminal investigation; you can’t convict people based on “our assumption”.
The document produced by the British side and circulated in Moscow to Foreign Embassies explained that Russia must be responsible for the Skripal poisoning because they have the “capability, intent and motive”. “Intent” means they want to do it. Again; this is a circular argument.
The British case that Russia did this looks very much like what it is. They’ve decided it must be Russia because it meets their current image of Russia. There’s not much else. A good barrister could tell you that this is precisely how miscarriages of justice occur. People get put in the dock because they “fit the bill”. This is what is happening here and the British government should have thought first.
They can’t make up their minds. In the text above the Foreign Office said “there is no other plausible explanation” that the crime was committed by Russia. Today they are saying, in connection with Russia’s attempts to call a meeting of the OPCW: “Of course, there is no requirement in the chemical weapons convention for the victim of a chemical weapons attack to engage in a joint investigation with the likely perpetrator”.  Well; which is it? “no other plausible explanation” or “likely perpetrator”. If they can’t decide in their own narrative between dead cert and “likely” why should anyone believe anything they say?