I got it out.

ended up drilling enough out of the head of the bolt that I could use a screwdriver and hammer a notch in the rim of the bolt, then it was just a matter of putting the screwdriver at the right angle, and hammering it around in a circle. It wasn't corroded, just in really tight.

I got the cover off and found that the chain had at some stage slipped off the front sprocket and chewed a hole in the alluminium gearbox housing, its less that 5mm long and 2mm wide, but it lets all the oil out. Thinking of filling it with solder...any ideas?