Cpt O'Sullivan's report is in the file M.I.9/S/P.G. (G) 637 (subsequently released by the British National Archives under WO/208/3307/28).
The report of Cpt O'Sullivan can be found in file M.I.9/S/P.G. (G) 637, which was later released by the British National Archives as part of WO/208/3307/28.