How do I remove A derwent macdee flushing chamber?

My toilet wont flush, the flushing mechanism seems to be housed inside a plastic chamber, it says Derwent macdee on a little blue lid on top. I cant seem to get access to this as it seems a sealed unit, any ideas on how I remove it to see what the problem is. the only way it seems it is accessable is by unscrewing under cistern, but as this is also connected to the toilet itself, and wont drop off the screws unless it is first pulled out of the toilet, its looking like I will have to remove the toilet from its base. any easier way anyone knows of?