Great Koromo ... thanks for this lead. I had searched in various places, but with little success. Margart apparently went under the stage name of Margaret Tremayne, or possibly Tremain, and was supoosedly an accomplished singer - some family stories even record her as an opera singer who appeared for the D'Oyley Carte (but I checked their web-site, with no record of her, so it's probably a myth).
Henry apparently used to describe himself as "the Original Silver King", apparently in reference to Herny James' well-known play "The Silver King". But the first performance of the play in 1882 featured the famous Wilson Barrett in the title role, so I'm not sure what he was on about!
Cheers
Tim