I've emailed Fiat, and sent this to the National Trust.

I cancelled my membership of the National Trust when I discovered that the Trust had accepted money from Channel 4 so that they could use the Uffington White Horse as an advertising space. My main concern, voiced in an email to the Trust, that was this would set a precedent for further company or product advertising at sites of beauty or historic interest. It seems I was right to be concerned. The beautiful Cherhill Down, owned by the National Trust, was despoiled this week by an advertising hoarding for Fiat Panda, and parts of the white horse were covered by black bags. Has the National Trust given it's permission for this? Are we to see more and more of this sort of thing? Do you have a list of sites where I can walk and not be bombarded by advertisements?