Looks like the Grimsby to Whitland fish train - it even has a LNER type brake van.

Peter Kerslake says "4074 is clearly fresh from Swindon Works, resplendent in new colours and doubtless recorded here on a "Running In" duty prior to returning to her home shed .
Although most such runs were made between Swindon and Bristol the line eastwards from the Works as far as Didcot or Reading was also used as seen here".