The 80/- that I brewed on Sunday appears to be fermenting quite happily, there is a 1.5" krausen, the SG has dropped from 1.052 to 1.034 there are small clumps of yeast whizzing around in the fermenter but it's not moved the airlock at all

normally I'd expect the airlock to be going mad by now! There is no where else for the c02 to escape, when I press the lid lightly the water in the airlock moves.
I aerated this one very well (using an aquarium pump and airstone) has that got anthing to do with it?
Why isn't it producing c02? during fermentation

anyone else experienced this?
(fermenting using Gervin English Ale yeast)