It looks like this is the one. I cut a section a short while ago and the internal measurement is approx 50mm. I didn't realise that sizes were given on internal diameters. You learn something new every day. Thanks to all who replied.
I am re-doing the kitchen in my coffee shop and have a problem with waste pipes. A coffee machine and wash hand basin run into 40mm waste pipe which runs along a wall for about 3.5m and then goes into what measures as 60mm waste pipe. There is also a double sink which has to be removed but the...