And it's not just NT that are to blame. Here's my letter to EH:

Dear Sirs,

Last August, following the vandalism at Uffington, your press office released the following:

"We deplore any unauthorised and therefore unlawful interference with the Uffington White Horse, which is a much loved local landmark as well as a nationally important and sensitive site. It is a Scheduled Ancient Monument and thus protected by law. Although in this instance there has been no damage to the horse itself, the surrounding grassland, which has been painted with white paint, is also part of the monument. The grass has now been strimmed and whilst it appears to have suffered no long-term damage we will be closely monitoring the grassland for signs of any lasting effects. Any unauthorised activity on a Scheduled Ancient Monument could cause damage to fragile and irreplaceable archaeology. The surrounding land is also a site of Special Scientific Interest which receives additional protection in law."

I note that you reserve your disapproval for UNAUTHORISED interference. The latest interference, of an apparently identical nature, was “authorized” in that it was carried out by the National Trust. What is your attitude to this action?

If you approve, are you fully satisfied regarding the “harmless” nature of the paint used? I understand the paint has now been hosed off and is now working it’s way down the hill in a still-visible mass. I am concerned, for example, as to whether such paint (described as lead free and non-toxic) is harmful to the larvae of rare chalkland butterflies. There are no doubt hundreds of similar ecological concerns. Do you have any plans to press for the removal of this residue?

Finally, I understand similar incidents may be planned elsewhere. Have you considered their implications, liaised with English Nature or formulated a policy?

Kind regards,