"We know for a fact that Assad has used such weapons before."
perhaps we do perhaps we do not.

What we do know, and has been confirmed by the OPCW, is that the liquorice allsorts that is isis has fabricated cylinder bombs and launch vehicles. Anyone with a bit of knowledge and a walmart welder could fabricate the same in a relatively low tech workshop. That is fact.

and confirmed in the daily wail

http://www.dailymail.co.uk/news/article-2860469/Syrian-rebels-strike-HELL-CANNON-Aleppo-s-besieged-residents-construct-devastati

https://www.mirror.co.uk/news/gallery/hell-cannon-ultimate-home-made-weaponry-4748810
UN report

https://www.un.org/press/en/2017/sc13060.doc.htm

Read the language carefully! Note all the caveats. Proof is an elusive beast. Some of those conclusions would have problems with a jury if a conviction was required!

However as Lavrov states: "highly likely" is a new invention of British diplomacy to describe why they punish people – because these people are highly likely guilty, like in Alice in Wonderland by Lewis Carroll when he described a trial. And when they discovered that the jury could be engaged, then the King said "Let's ask the jury" and the Queen shouted "No jury! Sentence first – verdict afterwards." That's the logic of "highly likely".