The thing which worries me is that TM. has now provoked a stand off in which nobody wants to lose face. To start throwing accusations and ultimatums around, and then expel diplomats based only on some circumstantial evidence is like burning all your bridges behind you. Why are we not allowing the Russians the samples which they desire ? There are many other possibilities here, other than a direct action endorced by the Kremlin - which are probably more likely. How can the Russians prove their innocence - are they required to do so ? To do this they would need access to the relevent materials. I would say that the onus is on us to prove their guilt, rather than the other way around. The problem is that with the escalating situation we have now, can the truth come out at all ? If the Russian government were to 'prove' their non complicity in this - would it be reported accurately ? Would there be an official apology from the British Government ? If ever we needed proof of the inadequacy of the British government, we are seeing it now - there is not a trained statesman amongst them.