With sacred wells it's a tough call. I think that each one has to be taken on its own merits. An old raggy tree is usually a fair sign of age, but there are some new ones around.

In Ireland there isthe luxory of checking who the well is dedicated to. If it's St Brigit then your looking at a pagan well. If it's the local saint then it's not so clear, but the day of the patron can be an indicator - is it near an equinox, solstice or other pagan festival. Bullaun stones by the well are another sign of pagan origins.

In the UK there is often only local knowledge and folklore to go by.

Wells at fords or bridges are often pre-christian. Offerings would be made to the water before crossing it.

Yep. Wells are tough one, but there is a high proportion of them that are of pre-christian origins, so if they are visitable then they should be included. Although one visitor may not see something about the site that screams 'AGE!' another might.

I suppose the best thing is for people to say why they think a well or spring is worthy of inclusion with details of folklore or nearby remains etc.

You're definitely right about one thing. A website dedicated solely to the holy wells of the British Isles could be massive.

2ps worth. Water is a vast subject, sea,river,stream and wells, but it belongs, at least in my mind with, stone and wood it did have significance in the past. Just by going to Goffik's Url on wells, I traced another piece of the jigsaw round here. Alphage well (Bath) had had water diverted by a farmer, from North Stoke. This water at North Stoke (marked as a spout) falls down a wall by the church, the church itself looks like an early saxon xtian site, situated on a mound, the stream probably wound its way down an old roman/earlier track, to the river avon below, where, strangely coincidental, the now disappeared , prehistoric stones would have been... water is an important element in the landscape, especially in Somerset, magically appearing and disappearing. These spouts have a similar effect to what happened at Boscastle, intense rain on the hills causes a cascade of water to follow a stream course and flood down the hills ..... of course you could say I just made up a story to fit the facts, but wells and water are clues and probably deserve a website on their own.
Moss

"Come up with a bit of evidence that they were 'sacred' to prehistoric people"

Fair do's. What constitutes evidence is the thing that needs sorted. Otherwise the presence of flint scatters could be used, which could be taken of evidence of nowt more that someone having been there with some flint. It wouldn't confer sacredness. The cloutie aspect is useful, but the inclusion of such places on a prehistoric list would have to be based on not much more than a hopeful implication of continued tradition, or at least continuation of the memory of a tradition. But before sacredness could be determined, would a defintion of 'sacred' be needed first?

I'm assuming a definition isn't needed for 'pre-historic', nor for 'people' ;)