Hello Peter.
I think you are wrong about the date I'm afraid, as the photos are of the "Castalia" of 1874. By coincidence I picked up "E.W.P Veale's "Gateway to the Continent" (Ian Allan, 1955) yesterday, and there is a less tightly cropped version of the second and third image you have posted in there.
If so, she was built for the LCDR marine department by the Thames Ironworks and had a pair of paddle wheels in tandem between the hulls, powered by four engines each with its own boiler room and funnel. Four seems to have been her special number, as she had four bows, each of which had its own rudder. She was not, according to Rixon Bucknall ("Boat Trains and Channel Packets", Vincent Stewart Publishers, 1957.), a good steamer and left the railway's service in 1884 to become an isolation hospital on the Thames.
Catamarans seem to have come in threes, as they also operated an almost identical vessel, but with only one paddlewheel and without the bell-mouths to the funnels, the "Calais-Douvres" (ex "Empress") built by Leslie's of Hebburn in 1877, and of course the infamous "Bessemer". None of them were particularly manoeuvrable, or good sea boats, (the Calais-Douvres being the best of the three) suggesting that she might make a better static than working model.
Having said that, I would love to see a model of her on the water.
All the best
Brian Gates
Gillingham UK