Drive gear on the Stihl hs45 hedge trimmer?

I am struggling to change the drive gear on my HS45 hedge trimmer. I have a new part from the dealer and have stripped casing down to bearings but cant get the sprocket thingy out as there are bearings there. The dealer wants £45.00 to do it but i cant afford that so i would like some assistance in knowing how it comes apart from here. Any diagrams or assisatance gratefully received.