I would say it's a gearbag out job. Take the starter out and have a look first. Thats only held in by 2 bolts and it's on the front of the case so easy to get to. I think there 6mm allen head screws iirc.
You should the be able to stick fingers into the starter hole and feel the bush, although I don't think i've ever heard of one wearing out. Whats the problem? if the starters sticking it's the solenoid, and that can be stripped out, cleaned and greased!
if you need a hand i'm only in coningsby so can take a look. or you can bring it to mine and i'll have a look!
Jon.