He was (almost certainly) wrong. Photography outside the National Trust houses is not generally a problem: I've done it many, many times with and without a tripod (e.g. here and here ) - the first of those was inside Lacock Abbey's cloisters so it would be a bit ironic if they were to prevent photography there (as the former home of William Henry Fox Talbot, it has the only room inside an NT house where you CAN use a camera whenever you like). Special cases aside, I have taken many shots of doors, wall sculptures and other exterior details without anyone being unhelpful. Certainly, the law allows you to photograph anything you like, except for obvious no-no's like prisons and other "Prohibited Places" (usually military) from the public highway.
Originally Posted by dferrie
Inside the houses however, photography is prohibited. The NT rely heavily on volunteers, some of whom seem to consider that the houses belong to them, personally... Although I think there has been an attempt in the last 10 years or so to wean out this attitude and become more "user-friendly", I do not doubt for a second that some of the "old guard" still exist (in fact I came across one in a NT car park about 2 years ago and got in to a big argument (non-photographic) over nothing very much (which was quite fun really: seeing how many shades of red he could go through).
Anyway, this has nothing to do with the OP, but I didn't want to leave people discouraged from shooting on NT property as the grounds can be highly photogenic and of historical significance. If taking a tripod go mid-week out of season and the jobsworths will not be able to complain that people might trip over it...
Have fun, Bob.