I like to pretend that was filmed in the Hogwarts sixth form commmon room in the eighties

It pretty much was. It was filmed in a Victorian sanitorium, https://en.wikipedia.org/wiki/Holloway_Sanatorium (built 1873-1885) which is in keeping with your (and JK Rowling's) vision of public schools, in particular Hogwarts was modelled on https://en.wikipedia.org/wiki/Fettes_College (built 1864-1870) in a similar architectural style.