We have an ancient well which is our water supply. It's about 20-25' deep, about 5' across and was hand dug into the rock. It is probably quite silted up but so far it supplies us and the granny flat, although we use captured rainwater for the stock and veg garden. If you use the water from your well then it will be tested and will have to pass purity tests - which it almost certainly won't pass. You then install a filter and UV light which is about £500 or so and everything is fine. I can't remember at this time of night who does the testing (at your expense) but if you phone your council they will have the information. Our well was already in use when we moved here, but I suspect that you would need council permission to begin using a well which had fallen into disuse.
To get the water out of the well you can use either an electric pump in the well itself, or have one in the house - ours is in the house which is noisy but at least we know when it's working or if it goes wrong, while the granny flat's pump is in the well (not submerged)
If you are going down your well do be careful - it is a closed space with the risk of gas. Also you need to be properly roped up and have someone on the surface.
There are water engineers who could clean your well out for a price, but it might be possible to do it yourself, with safety precautions as above. Once cleaned out you would need to wait for the sediment to resettle before you could use it. You also have to have suitable housing above the well, both for safety and to stop muck falling in.