Hello and welcome Marinade.

I haven't read all the responses above, so please forgive repetition. There are several examples of "cloutie" trees around Cornish healing wells (if you have a look at TMA entries for Madron and Sancreed, there are several pictures). Like Bladup says, there seems to be a water connection. This also seems to be a long-standing tradition there, I'm not sure if the ones near West Kennett are so long-standing?

I've also seen them at Doll Tor and Nine Ladies stone circles in the Peak District and at Harold's Stones in Monmouthshire. Of course in the Peaks they also "dress" their wells in annual festivities, but the decorations are removed afterwards, which is rather tidier than leaving pairs of pants and old rags ties to a tree until they bio-degrade (or don't).

My own view is that it simply adds unwelcome tat but, as has been said above, there are plenty of different points of view on that. The main issue to me is that it starts to extend to include other stuff, plastic flowers, teddy bears, etc, etc. Sooner or later the place starts to look like a landfill.