The part of Henri Rousseau is played by James Lloyd, not the small-part actor of that name, but a real-life artist who worked as a railwayman during the week and created elaborate paintings (in the "pointilliste" style) on Sundays. These had been acclaimed and exhibited and Ken Russell had made a documentary film about him for the "Monitor" series, so when he later moved on to making a drama-documentary about Rousseau, he thought of asking Lloyd, as a sort of British equivalent to the French artist, to play the lead.
Although this Ken Russell TV film is always referred to as Always on Sunday, the title card on screen (per the DVD version on the BBC boxed set) actually reads: Henri Rousseau - Sunday Painter.
This film caused a small controversy when it was first screened as it cast a woman - Annette Robertson, who was Ken Russell's mistress for a time - as a man, the painter writer and Dadaist, Alfred Jarry.