Show me or shut up.

On TV news yesterday morning, Boris Johnson, the Foreign Secretary (I still find that coupling hard to accept) assured us that "the evidence is overwhelming" that Russia was behind the poisoning of Sergei Scripal and his daughter.  So that's it?  He says it, so it must be true?  It may be, but I'm afraid I need more than his word for it.  Let's see this overwhelming evidence, Boris.