As you guessed it's a still of Beryl Reid, playing the lead in the film 'The Killing of Sister George'. She's dressed as Oliver Hardy because her and Childy/Alice, played by Susannah York, went to a fancy dress party as Laurel and Hardy.
The party was filmed in the famous lesbian club the Gateway Club in Notting Hill. Incidentally the Dutch Countess I used to work for as a student, was an extra in the film, and can be seen in the film doing the twist.
