The tie is the Gordon Highlanders regimental tie, and the tartan appears to be the Princess Mary Tartan (based on Royal Stewart with a dark green ground in place of Red) -- a graphic is found of it on the "Royal Tartans" page on the Tartans Authority site:

http://www.tartansauthority.com/Web/...yalTartans.asp

Third one down on the right.

Charles is very proud of his association with the Gordons, who were amalgamted in 1994 with the Queens Own Highlander (Seaforth & Camerons) to form the Highlanders.

A very nice photo -- thanks for posting, Martin, and best wishes to them both!

Cheers,

Todd