Hello Chrisgat,
Welcome to the forum.
That's a very nice cover indeed.

The postmark is from the receiving house in
Hanover Street in Edinburgh.
It was addressed to the
Earl of Traquair, Traquair House, Peebles which is about twenty miles from where I used to live.
THe house is supposed to be the oldest continually inhabited house in Scotland. Here's a picture from Wikipedia:

This letter was sent to the eighth (and last) Earl and the house passed to a cousin when he died. It is still owned by a member of the family.
He house is famous for its gates which are not to be opened until a Stuart monarch is back on the British throne.
It also now produces very good specialty beers.
Here's the house's own web site:
http://www.traquair.co.uk/