The Twycross 40th Anniversary booklet states that the orang house was built in 1973, and the gibbon complex in 1979. It also states that the original gorilla house was built in 1980, that is the one near to Norton Grange, I always thought it was older than that.
I guess they know their dates...I cannot remember when I first saw the Gorillas in their 'new' house but I think it would have been after the younger ones(Biddy & Eva) arrived, soon followed by the two males(Mamfe and Assumbo) from Jersey. They were born in 1973 so that date would fit pretty much.