@leocg Apologies - I disabled my extensions and cleared my cache and the videos didn't autoplay - although it looked like the video was downloading in the background judging by the bandwidth used.
It seems the adblock plus extension is allowing the videos to autoplay. Unfortunately, adblock is pretty much the most essential extension to have an acceptable web browsing experience, so disabling it doesn't seem to be a good solution. I'll send them a message to see if they can help.