In the 1669 visitation (listed in the appendix of the visitations of 1566, 1382 and 1634, pub Harleian 1884), pedigree 18 was for Corbett of Sundon.  Subsequently this was transferred as follows: “Mr Cheyne of Sundon to William Clayton, Esq” (p. 208 of the Beds visitation).  I wonder if this was somehow later transferred to James Buchanan.
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It seems, from a book “The Buchanon Book” by Alexander Buchanon QCof Montreal. published privately in Montreal in 1911, that James Buchanon was of the Buchanons of Drumhead.  He had an elder brother Archibald who inherited the Drumhead estate and had three sons and six daughters.  James is described as an eminent merchant in London and “left an only daughter who married Sir Walter Riddell of Riddell, Roxburghshire.  Her son, Sir John, on succeeding to the estate of Sundon in Bedfordshire, added the name of Buchanon to his own as required by his grandfather’s deed of entail.”  Regrettably Jane, the daughter of James B., does not seem to have married Walter but instead married Sir John, per Burke’s Peerage. and confirmed by their marriage settlement.
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Or a lion rampant Sable holding in his paw a man’s heart Proper, within a double treasure flory counter-flory of the second.

“An Ordinary of Scottish Arms” by James Balfour Paul, pub Wm Green & Sons, Edinburgh 1893, p. 170.