Information from J Thickitt:

The Master Cutler was the name of the train and 1B20 the Train Reporting Code; '1' indicated an express passenger train, 'B' the destination London, King's Cross. Therefore this was the 'Up' service, calling only at Retford. The diesels hauling this train were shedded at Finsbury Park Diesel Depot in London; usually (but see below) this was one of their Brush Type 4 locomotives, as here. By the date of the photograph the first 50 or so of the class were at this depot, so the number of the locomotive shown would have been in the D1500 series.

Notes:

1) The Master Cutler was a Pullman service, by the date of the photograph still providing first and second class accommodation. First class Pullman cars had names, second class had numbers. (One of the cars often present in the Master Cutler formation between June 1964 and August 1965 was 'Eagle', which is now in the National Railway Collection).

2) between the morning and afternoon Master Cutler services (during the middle of the day) there was a further return train called The Sheffield Pullman, the same cars being used for all the day services.

3) the locomotive was replaced once each day, if remembered correctly this was after the train's morning arrival at King's Cross.

4) between the above dates sometimes the train was hauled by 'DP2', an experimental diesel, a Type 4 built by the English Electric company.