He was good in it. He had a big part as an old friend of Poirot. Art Malik (Doctor in The Professionals episodes 'Rogue' and 'The Rack') was also in it and was good in it. Of course Poirot himself was in The Professionals episode 'Where The Jungle Ends'.
A while ago I saw Paul Herzberg (Serpoy in The Professionals episode 'Mixed Doubles') in the Poirot story 'Dumb Witness'. He also appeared with David Suchet in the Tales Of The Unexpected episode 'A Time To Die'.