You're right, Richard- the only Jamie Wiseman-Clarke I could find online is a partner at Bryan Cave Leighton Paisner LLP, b. 1976 according to company records; I found a 1976 birth record for a Jamie Neville Wiseman-Clarke, mother Nash. He has an elder brother, Thomas Charles, born 1974.
'N. G. F. Nash' would appear to be Neville Gresham Fountain Nash (1911-1982), m. 1947, Thora Marguerite E. Thorsson, which would corroborate the above as providing Jamie's middle name. A 1965 list of members of the Institution of Civil Engineers lists him as of Kenbane Cottage, Ducks Hill Road, Northwood. He was son of Maj. Fountain Okey Colborne Nash (1878-1915), of the 5th Battalion, Northumberland Fusiliers, employee of the Hertford and Rochdale engineering departments, surveyor and water engineer of Cockermouth, assistant to the city engineer of Newcastle-upon-Tyne,and chief planning assistant to the City Engineer of Birmingham. He married Ella Heap Handley in 1905. and was killed near Ypres.