The first batch of twenty Class 20s were among the very earliest diesels on the national system in the 1950s, many were allocated to the first diesel depot in the country at Devons Road, Bow, in East London (long closed), which was actually a London Midland Region depot despite its location, but a few were at Willesden. They dieselised, all in one go, the freight workings from Willesden Yard round the North London line to the London docks, where the steam depots had been suffering significant staff shortages. This traffic fluctuated considerably from day to day depending on what ships were in port or not, so when not so required they were used on Euston ECS and local passenger workings.
It may seem surprising, from today's view, when a 3,300 hp Class 66 is sent out even if there are just a couple of wagons to haul, that a 1,000 hp diesel loco was assigned to full length freight or ECS trains, but they handled these just as well as the 'Jinty' 3F 0-6-0Ts they replaced. The low power will start a large train, and get it up to a certain speed, the extra horsepower is only required for the combination of weight and speed. The freights on the North London line didn't often even get up to 30 mph, and they were fine with that. As far as I recall they were never run in pairs until later and elsewhere. The London docks closed very rapidly in the 1960s and the locos moved away from the area.
I believe these locos had no steam train heating boiler so would only be used on passenger trains in the summer when no heat was provided.