Peter, thank you, I measured it and then forgot to add the size to the post!
It is 8¾ inches across (flat side to flat side) and 9½ inches across (from point to point). The pattern is on the underside, the topside is very shiny smooth and reflective which is why it looks vaguely opalscent (it isn't really). The second pic shows it held up to the window (my blurred fingers top right) with the unfrosted side towards the camera. The frosting is fairly smooth, satin rather than silk, but not rough and harsh. I feel it's not English, but I couldn't come up with where I thought it may be from. No idea how old it is either.
Guest is right, the pattern is very sharp but I think it's moulded rather than cut. The diamonds are noticeably pointed and crisp.