Dave's link,

"Gary Aitkenhead, the chief executive of the government’s Defence Science and Technology Laboratory (DSTL), said the poison had been identified as a military-grade novichok nerve agent, which could probably be deployed only by a nation state.
Aitkenhead said the government had reached its conclusion that Russia was responsible for the Salisbury attack by combining the laboratory’s scientific findings with information from other sources.
The UK government moved quickly to make it clear that the prime minister, Theresa May, had always been clear the assessment from Porton Down was “only one part of the intelligence picture”."