A kind ex-installer donated the requisite replacement part - a foam insert instead of using a bolt. All we had to do was lift the turbine shaft up, slot the foam insert in place and drop the shaft back down.
We had an impossible job getting hold of scaffolders at a price which justified the work, and eventually decided to build our own scaffolding using 4.8m lengths of timber and lots of cross-members.
The scaffolding was quite a simple affair, with a plywood platform at the top on which to stand a small ladder for the final climb to the turbine's wall brackets.
The wind turbine shaft was very tightly secured - that's a good thing! - so was hard to manoeuvre the shaft around and get the foam collar into place. After a long while of unbolting, crowbarring and heaving (all at a height of 6m) we finally got the shaft straight and the collar in place.
We wouldn't recommend doing this job yourself if you have the faintest fear of heights or if you value your life.
The screws pictured are all that we used in the scaffolding; probably not enough, by the looks of it!