I can't answer your question about cost William, but we have considered the same thing here and ran into an immediate snag. The French regulations here for septic tank systems, which are more stringent than the UK, specify that a well cannot be closer than 30 metres to a soak away system. As ours would be only 10 metres the idea has been abandoned. Don't know if that regulation applies to you or if you would be affected?
The difficulty with yours, as ours, would be getting the cap off in one piece, without bits dropping down the well. Then you may also find that the original wall, the roof and all the winding gear was dropped into the well so would all need removing.
So getting an idea of cost may be difficult without further consideration/ examination.