But - is there another way of looking at this affair? I believe there is, but it takes a leap of the imagination, and I have no proof of my theory (I had to get that out of the way).
Let's start with one indisputable statement: the Russian secret services are very good at assassinations, and quite good at disguising their handiwork. Wikipedia lists a number of such killings, both in Russia/USSR and abroad. Victims include: Leon Trotsky: Stepan Bandera (Ukranian nationalist, murdered in West Germany, 1959); Georgi Markov (The KGB used the Bulgarians - allegedly); Alexander Litvinienko, killed in 2006. Most such murders were KGB operations, but more recently, the FSB, successor to the KGB, has taken up the mantle with mixed results (the FSB hit on Alexander Litvinienko was not exactly a model of efficiency).
It hardly needs to be said, that if the FSB is prepared to carry out killings abroad, it is unlikely to have reservations about assassinating dissidents in Russia itself. It is undeniable that a number of critics and opponents of President Putin have met violent deaths. It is also undeniable that, as far as President Putin's involvement is concerned, nothing has been proved (cue Dusty Springfield). Mack the Knife, as we know, always wore white gloves.
As an example of Putin deniability, we can see it in the murder of the journalist and bitter critic of Putin, Anna Politkovskaya. After years of publishing books attacking Putin and receiving harassment from the FSB, she was found shot dead in the elevator of her apartment block in central Moscow on the 7th of October, 2006, which, coincidentally, was President Putin's birthday. She had been shot four times in the head. Arrests were made, and several Chechens have been convicted of the murder - but the instigator, the man who ordered the operation, has never been identified. What puzzles me about this case is the fact that the FSB monitors foreign diplomats, journalists and domestic opponents rigorously. That being the case, Politkovskaya must have been under surveillance at the time of her murder. It follows, then, her assassins must have been observed by FSB watchers - and yet, incredibly, the murderers escaped. Even after being caught following a long and laborious investigation by the Russian police, they have not named the instigator, certainly not President Putin.
Hopefully, whoever carried out the attack in Salisbury will be caught. If that happens, it is extremely unlikely that the culprits will have any direct link to the Kremlin. If, hypothetically speaking, Putin ordered this hit (and others), he would have used "cut-outs". That is to say, he would have relayed the order through a succession of people "A orders B orders C orders D, etc", which would make it very difficult to trace back. It may be that the putative assassins have already fled the UK, as did the confirmed killers of Alexander Litvinienko. That being the case, they will never stand trial here, or in Russia. No wonder opponents of Putin living in the UK are nervous.
So - do I believe the UK government have made a good case against Russia over the Salisbury poisoning? Er, no. But - do I think the Russian secret service smart enough to plan and execute a hit without leaving evidence of involvement? Yes. Do I have evidence of this that relates to the Salisbury chemical attack? Er, well, no.
Like Dusty sang: nothing has been proved.