It was diesel to overhead. It was the Thameslink train and I think it used diesel south of London (to Brighton).

When the snow hill tunnel re-opened in 1988, and trains started from Farringdon to Blackfriars, they were operated by class 319s, which were dual voltage from the very start. While I'm sure some diesel engineering trains have been through the tunnels, and perhaps some diesel goods trains in the 60s before the closure, I'm not aware of any diesel passenger trains through them (as there were no passenger trains since 1916)

Some 319s are being converted to partial diesel and becoming class 769s, but not on this route.

Oh it could have been switching from overhead to third rail... I never actually went on the train to Brighton, I just remember the pantograph coming down.

Still does it now, there's a similar switchover north of Shepherds Bush. The Southern 350s used to stop when I got that train, I think the overground trains can raise/lower on the fly. I assume there's a similar changeover in East London.

Before 2009 and HS1 to St Pancras, Eurostar also switched from overhead to third rail, on the fly I think, somewhere near Ashford.