3210 - Photo: Paul James.

55021 (D9021) "ARGYLL & SUTHERLAND HIGHLANDER" passes Hornsey Depot on the 10th July 1981 with 56023 in tow. The pair were en-route to Stratford TMD for the open day there on the 11th July.
The class 56 most likely 'picked up' at Doncaster and judging by 55021's appearance it is assumed Stratford were left to clean the loco and paint the yellow ends themselves - see here.