40050 looks ex-works so must be either November 1950 (Heavy General) or May 1954 (Heavy Intermediate). On both occasions 42518 which also looks to be recently off works, had been in works 6 months previous, May 1950, October 1953, so that will not assist dating photo. Both locos received overhaul at Bow Works.
Graham Ledger pointed out the way the tank lining goes up (rather half heartedly) in front of the cab cut-out - we have never seen that before. Suggests the earlier date with Bow Works not yet used to BR lining!