Essentially you can now use a movie or still camera at the discretion of the property manager. Flash is still forbidden because it annoys other visitors. Tripods may be permitted when there are few other visitors. I doubt if movie lights will be permitted both because they may annoy other visitors and because some old fabrics might suffer from too much bright light.
This news may not have percolated to all NT sites, so tread carefully but ask if you want to shoot and point any doubting managers to the "Visitor FAQs" on the National Trust website: http://www.nationaltrust.org.uk
Well done the NT for being willing to listen to the arguments."Amateur photography (including filming) without flash is now permitted in historic interiors at the Property Manager’s discretion. As with outdoor photography, any photographs taken are strictly for private use, and enquiries about selling or publishing photographs should be directed to photo.library@nationaltrust.org.uk. However, visitors must be aware that at some places, there may be copyright issues, and further permissions may be required in respect of collections not owned by us. In these situations the Property Manager decision as to if photography is allowed is final. "
-Dave