I travelled from Reading to Penzance in September 2000 behind a class 47 (I might not have noticed if it had been a 57, but Wikpedia indicates that they hadn't yet been introduced on GW at that point so it must have been a 47). The train included one or two Motorail vans at the front, and I think it ran every summer Saturday. It was a morning departure, though (maybe about 10.30 from Paddington?), whereas the OP mentions leaving in the afternoon.
It was still running in summer 2002 -- I remember seeing it preparing to leave Paddington. For what it's worth (it doesn't cite a source), the Wikipedia article on class 57s says 'It [57601] hauled Mark 2 stock on day services from London Paddington to Plymouth and Penzance—until these ceased in September 2002...'.