There are two pages on the British Caledonian tribute website devoted to this ferry trip:
BCal Spirit of Caledonia . I think I saw a documentary on TV (Sky?) some years ago about the flight across the Atlantic but I can find no reference to it.
Interesting that the BCAL website says OH arrived at Glasgow on 20/5/75 but the Strathallan arrival date is shown elsewhere as not until 11/6/75. RAF Museum records show that the Cosford Nimrod XV249 escorted OH on the final part of the Atlantic crossing on 20/5/75. Must have spent three weeks at Glasgow?