MacRobert's Reply
Just had a look at 'Gifts of war' and can only find a single Lancaster listed, 'Malton Mike' Serial number KB999 which was donated by the staff of Victory Aircraft of Malton Ontario.
The aircraft donated by Lady Rachel MacRobert were indeed Short Stirlings as has already been stated in this thread; they were serial number N6086 and W7531 which were operated by 15 Squadron so there is still a possibility that there was a Lancaster which carried the MacRobert name.
From December 1943 until March 1947 15 Squadron operated Lancaster Mk1s and Mk3s and then from February 1947 until October 1950 the used Lincoln B.2s which are developed from and outwardly similar to the Lancaster so it may be that the squadron decided to continue the tradition of having an aircraft with MacRobert's Reply marked on it. If you are looking for more detail, the next place I'd go would be a history of 15 squadron (if such a thing exists).
Happy hunting...