Nice! A 1954 engine, for those for whom age is important... also formerly a Start-o-matic unit, which one can tell from the combined fuel-rack-closer/decompression lever arrangement, originally this would have been operated by a solenoid. I'm not sure if, at that age, Lister were indicating whether the engines were radiator cooled or not; if they were, then this engine was originally tank cooled (no "R" for Radiator in the serial number).... but that might have only been 1970-onwards. The other clue to this originally being a tank-cooled model is the lack of fan pulley on the inside of the radiator side flywheel.
Love the "fuel tank"!
Good luck with the sale, wrong side of the pond for me unfortunately, she sounds very healthy!