Accusations will damn a country/person before any evidence is ever provided, it's the way Western media works these days.
Once again, with all these "attacks" something stinks about it, there's a lot of missing links and in this instance Jeremy Corbyn is spot on.
The fact that the UK refused to send Russia samples of the chemical and instead rushed into a frenzy giving Russia a 24-hour deadline says a lot IMO. The next few moves should reveal more of the motive.