A strange case: Without concrete evidence, the whole emphasis of blame has been placed on the Russian state.

But why would Putin break convention, which is not to kill a swapped spy? They've never done it before, and the swap system is unlikely to be abused by 'diplomats' as it allows them freedom when they get into tricky situations, for example get caught out breaking the laws of the country in which they hold immunity.

This botched attempt at assassination does more harm than good to Putin (presidential elections in two days time, World Cup hosting in a few months), so the question I'm asking is, who would such an assassination benefit?

Litvinenko wasn't a 'swap spy', although parallels are being drawn: he was a defector. I've very little doubt (but still some!) that his murder was state sponsored with Putin at the heart of it. However, Litvinenko was heavily and personally involved in implicating Putin in Russian Mafia State involvement. Skripal was small fry. He was only sentenced to 13 years by the Russians in 2006, a very light sentence compared to many compatriots found guilty of treachery by the Russians.

The evidence provided so far, and feverishly regurgitated by the media, is that Russia produces the nerve agent which has severely damaged Sergei and Yulia Skripal. Many countries make novichok derivatives, including our own, and Israel (for Mossad use). In fact the UK, using shared technology provided by the Russian state following the end of the Cold War, produces novichok and other chemical weapons (for research only of course) at what is often mistakenly called Porton Down. The chemical/biological/radiological/nuclear facility is actually at nearby Winterbourne Gunner.

And here's the important point (it's taken fucking ages to get here): Winterbourne Gunner is near Porton Down. It's also 4 miles from Salisbury.