I like Robert Fisk, he is hell of a solid correspondent but on this case he is absolutely wrong.
The Douma gas attack has been investigated not only by the OPCW, but by organisations such as the UN, the Red Cross and pretty much all serious journalism from the BBC, NYTimes to The Times and Wall Street Journal. They all agree that the Syrian Governmant gassed its own people i Douma.
The consensus is overwhelming, the wiki page on the incident lists 119 sources. I obviously haven't checked all of them but most look credible to me.