I was able to undo the two plastic 'daisy' screws, but the other two have the screw heads pressing against the metal parts of the firewall, so I can't get to the heads to unscrew them - unless they unfasten using the two plastic discs around the screw threads.

As for the rubber gasket - it may be glued on, as I've not managed to remove it completley. The manual claims that it's in a single elongated 'U' shape around the wiper motor (not the three seperate pieces like the pre 04/93 cars are), but mine completley surrounds the motor and there doesn't seem to be any obvious way to get it off.