This fits with my recollection. I was only 7 at the time so clearly a sketchy memory at best but. I'll have to search harder for the timetables.In the Public timetable HSTs were booked on the Cornish Riviera and Golden Hind in each direction from 1 October 1979.
But, according to Railway Observer October 1979, this actually started on 6 August 1979.
Helford would be my guess, based on my memories of the approximate route we took and having just looked at some maps.The small ferry could be either the Falmouth to St Mawes or the Saltash to Plymouth..... !