National security matters and indeed media reporting are held to a different standard than domestic felony cases. The best that can be achieved is a balance of probabilities.Not really, Russia has asked for a sample so they can check the validity of the accusation. We've ignored that entirely justified request, so by refusing to supply the evidence of guilt we have in law conceded they are right and are innocent.
Our CPS and police forces have been currently under fire for exactly the same, failing to disclose evidence, thus calling into question many cases.
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Diplomates serving in another country are there as guests of the host country and may be asked to leave at any time. The only right they have is not to be detained or have their belongings searched... Although even that right can be resinded by their ranking superior the Ambassador. Case in point such as a member of staff being involved in a RTA. Mr May was fully inside her rights to request any or all of the staff including the Ambassador to leave. However it does invite a response in kind.