It probably doesn't help anybody but I'd guess that the photo of Phil Vincent and Montgomery was probably taken at one of the early '50s Earls Court Shows. If I remember correctly Monty was there as a VIP guest and BSA gave him a brand-new Sunbeam S7 (because he had owned a Sunbeam some years ago I gather). I'll sift through some of my books and see if I can find a direct reference to that show and/or that photograph.
Mark, all I can help you with is D.G. Lashmar's race results for his TT races:
1951: Lightweight, Retired, Rode an L.E.F.
1951: Junior, Retired, Rode Norton
1951: Senior, Finished 13th, Norton
1952: Senior, Fin. 30th, Norton
1954: Lightweight, Retired, L.E.F.
1954: Junior, Fin. 32nd, B.S.A.
1954: Senior, Fin. 21st, B.S.A.
He clearly enjoyed his racing! These figures come from '50 Years of T.T. History' published in 1957 for the Golden Jubilee of the T.T. Unfortunately I can't tell you any more about Mr. Lashmar, but good luck with your research and publication; I look forward to reading it.