If you draw the drinking water separately, into jugs say, and only treat that, then the contaminated water is safe for all other uses, even cooking, since boiling will kill the bacteria. There are any number of systems, both commercial and homemade, that will sanitize the drinking water. A solar cooker, a glass jug set out in the sun, various chemicals including bleach, sand and charcoal filters....all of it works.
The larger issue is to try to determine how the bacteria are getting into the well. This may be because there is no pavement around the well-head, and so surface water is trickling down the outside of the pipe....this is easily solved. Other than that you have to look into how far is your well from sources like septic fields, areas where manure is piled, and such like, and try to remove or remediate these if they are nearby. Once this is done a one-time "shock treatment" of bleach poured directly into the well may solve the issue for good.
The source is probably animal faeces, but we don't have control over the land which has runoff towards the land. We want to use the water through a hose (maybe jet washer) for washing foodstuffs, so need to maintain reasonable pressure.
I was hoping someone might have experience of a particular filter which I was intending to buy and install . UV has been mentioned, but would charcoal or sand work which is easiest to manage etc
The topology is such that a stream runs right past the well, and the runoff to the stream is from other farmers fields. I cannot see a feasible way to stop contamination before the well.
We don't want chemical treatment, and some other local residents / farmers have said just use a filter with cotton and activated charcoal. We want to use the water for washing fruit so want to make sure that it's drinking standard but don't want to taint it with chemicals.
Some people have said that a relatively cheap UV system after physical filtration will give us potable water, I just hoped that someone had experience they could share.
You might want to look into ozone systems. On a really tiny scale UV can work but I personally would not want to rely on it in your situation. Even an inexpensive ozone system can treat thousands of gallons a day.
I built my own for ozone system for a few hundred dollars. It is simply an ozone generator (you'll want a UV rather than Corona Discharge) connected to an air (aka aquarium) pump with a diffuser stone in a storage tank. The tank can sized according to how much water you need per treatment cycle. It can be as simple as that if you have no need to filter. In my case I'm removing iron and manganese, so I also have a recirculating pump to catch the crap that falls out of solution after being oxidized by the ozone. You can also get more complicated by adding a venturi valve, which is more efficient than the diffuser.
Ozone has been used for municipal water treatment for ages.
I had good results with a slow sand filter I built for real cheap. there weren't any microbes to begin with, so I can't speak to that with personal experience, but slow sand filters have a long record of effectively removing coliforms and other pathogens.
simple plans are easy to find online. a search for biosand filter is a good place to start.
When we bought our house we had the water tested as part of the conditions of purchase. Coliform was found so the previous owners tried the shock treatment on the well but ended up having the UV light installed. We have to replace the light bulb once a year.