I would concur about making mistakes in one's youth (personal experience etc), BUT, the OP mentions 'Pinza' in particular, as opposed to remembering just 'a Deltic', so did they witness the nameplate and/or number (D)9007? Deltics did visit the DRS at Stratford, and a couple of books I have, D.Brennand's 'London's East End Railways - part 1' has a photo of 9007 outside the DRS in March 1972 (page 31) and another of 9000 inside same in July 1970. Both taken by an Arthur Nugent, known to the author, as both worked there. Also in 'Diesels Nationwide' by Keith Montague plate 144 shows 'The Black Watch' approaching Stratford station from the South West (light engine) heading towards Temple Mills in 1976. All later dates wise than the OP's question I appreciate, but obviously the class did have a presence on that patch other than for open days at 30A.