Actually, I checked and this is NOT TRUE.
Lord Montgomery had two wives. The first died in 1965 and they only had one son. The second was Tessa Browning, daughter of Daphne du Maurier and they have no children.
Here is the entry in The Peerage for Tessa Browning:
http://www.thepeerage.com/p21865.htm#i218643
Further, the MacLeod estate is owned by Hugh MacLeod who is the chief of that clan.
It makes me wonder if people aren't making things up in order to try and sabotage her current popularity.