"I bought the front sprocket off ebay you linked - is it easy to do?
Also what purpose does it serve to the one already there?"
1: As others have mentioned, yes it is easy, but you should replace the lot as a set because old chains chew new sprockets away quickly, and old sprockets do the same to new chains because they already have a wear pattern in them. Chains are a harder change than front sprockets, though still not beyond a back yarder like me with the right tools.
2: The replacement is meant to spread the contact area of the sprocket on the shaft across a greater area, to prevent the erosion of the shaft caused by an over tight chain pulling the "floating" sprocket sideways against the shaft when the suspension compresses and pulls the chain tight. If the shaft isn't worn then you don't need a "super sprox" mod, but most are because idiot mechanics and previous owners haven't set the chains correctly on those bikes.
If on the other hand you've bought a genuine Honda front sprocket, the difference is almost zero as the pic of my bike's shaft will show. Chain slack is the saviour of healthy counter shafts not special sprockets, which are for prolonging the life chewed up shafts.
