its very awkward! had a new battery fitted on mine last week, watched the bloke fit it and it took him about 20 mins. (remembering he does them all the time!)
you need to take cover off and the rubber seal in front of the battery, then down the gap in front of the battery there is a screw which you need to loosen (that was the awkward bit hard to get to and work with) if you manage that the battery should just lift out.
good luck, but myself i would just go to halfords and pay a fiver for fitting.