All human life is here - from Paul Waugh's Evening Standard blog. The Mail has a longer but less juicier article with a photo of the alleged individual.
When I first heard the news item about CS gas, I thought perhaps PC Plod had gone over the top (what's wrong with the good old fashioned truncheon?). On reading this I am not surprised the officer discharged said gas. The altercation was within hearing range of the Commons chamber (Kerry McCarthy MP said on Twitter: "Was in Chamber when Pickles' guest went on rampage. Much excitement, banging + shouting. Gillie Merron told us someone had set off CS gas."). If the Speaker's chair end of the chamber is anything like the other end, they have two officers with machine guns at the doors (when I once strayed towards the Commons' entrance thinking it was the way out, they were immediately on the balls of their feet, twitching their guns, asking "Sir?"). So on the face of it, it is not surprising that the officer acted so forthrightly.
With a large hat-tip to the tweeting James Graham.