They were the normal type for the Southern's diesel operations across the region. However it was only this route that ran such lengthy formations. Bizarrely, although the operation from East Grinstead was basically a London outer suburban one, they were based at St Leonards depot in Hastings. Every morning after the peak a 9-car ECS took three of the sets on a lengthy run from Victoria down through Haywards Heath, Lewes, and Eastbourne (reversing there once the triangle cutoff was closed) to St Leonards, where they were refuelled and/or changed over, a return service operating after lunch in time for the evening operation from Victoria. I used to do some work in the council offices overlooking the line at Lewes, and the noise as they opened up past the station was audible right at the back.
Slowly over time, as other diesel services closed or were electrified, more and more units came to this line to reinforce the peak service. The peak Class 33 loco hauled services, with Mk 1 stock, ran to the end from London Bridge, I think they were based at Selhurst, about three sets and four locomotives. The first evening train ran up top-and-tailed, the loco that brought the first one in then shunted out and took the second one out, etc, the loco that brought the last one in returning light. Don't know how they did the same operation at the country end.