How do I remove a keyless chuck from my Sea...

The chuck is screwed on with a normal thread, the screw in the centre (already removed) is left hand thread. Holding the drill with the open end of the chuck facing you, it should unscrew anti clockwise. It might take an impact to start it off. I normally put the gearbox to slow, set the cluth to drill and then holding the drill firmly, hit the allen key against the edge of my workbench. Bring the drill down like a hammer blow so the allen key hits the bench (which should be on your left). I find this more effective than holding the drill on the bench and hititng the allen key with a hammer as it allows for less drill movement at the point of impact.