Thanks Bill, that's very useful: It sounds like it's the same as mine (80 OD, 70 ID), so if they call that 75mm, I know what I'm looking for. The quoted size seems to be an average of the ID and OD (probably not, just a convention of some sort), which is interesting. As you will know, these have to be compatible to work together, with relatively little scope for bodgery, given the way the socket is integral to the pipe and the nature of cast iron. Unlike the plastic stuff, it's not a cheap mistake to make, as you can see from your invoice (which is a very good price from what I've seen).