Dave,
The experts say they cannot confirm who manufactured and administered the agent

They can but have not the power to state it.
The Guardian article you quoted,
"The OPCW does not have the power to identify the source of the nerve agent, only to spell out its chemical properties. It is standard OPCW procedure not to identify the laboratories involved in testing the samples, but the organisation draws from a multilaterally agreed list of labs."

Even your Diane Abbott says it is "beyond reasonable doubt."