This is an insightful article written by a former British Intelligence offer who has worked in the Middle East which shows the sort of due process that intelligence goes through to confirm what has likely happened and where the weapons we stored and collected from. This is the sort of information that the UK, US and French Governments will have used before launching their attacks.
http://greyharemedia.com/intelligence/I've built up quite a big Twitter network of key players or former key players in the intelligence, armed forces, politicians, academics, analysts and journalists over the last 4 years who deliver, cross reference and corroborate stories and I trust. One of them is the German journalist Julian Ropcke who works for German newspaper Bild and has covered Ukrainian & Syrian wars. If you want to see the aftermath of the Douma attack. But be warned the pictures are very graphic and potentially upsetting. His twitter account can be viewed here:
https://twitter.com/JulianRoepckeWhat this does clearly illustrate is what a tool / useful fool Corbyn is and how much he is allegedly owned by Russia and Iran.