I did not know Hymeks worked into Kent - there is nothing on Six Bells so presumably this is a train (1X06) from the WR or possibly from the Midlands heading through to Margate/Ramsgate?
Robert Waterman says "As this is at Seasalter, it will have needed to have come from wherever on the WR via the West London Line and joined the LCDR Victoria - Ramsgate line at Factory Junction (only about 2 1/2 miles from Victoria). Whether the SR provided a pilot to a WR crew or whether Hither Green men were trained on Hymeks (they would have known the road) I don't know. "

Keith Morris suggests " Probably a "Mystery Tour" from the WR. (Hymeks were often used on Mystery Excursions from South Wales) - we ended up in all sorts of places! "