Does anyone know whether cast iron downpipes (from gutters, not soil)
are measured on the internal or external diameter? I need to repalce a
short run, and bit I have is about 80mm external diameter (more or less,
accounting for the paint and rust), while all the CI pipe I find online
is 75mm, so I'm wondering whether I should be measuring the ID or
whether the new stuff is actually a different size. (Of course, it
would have been either 3" or 3 1/4" in the day.) Just to confuse the
issue, some suppliers have plastic pipe in 80mm, which may be a
cost-efective way forward (as 6' of CI seems to be £70-80!).

If it wasn't 20 feet plus up the wall and above the conservatory roof,
I'd rip it all down and replace with plastic, despite the historic
aspect, but that will have to wait until we have some scaffolding up for
the roof job, which will probably be next summer now. I don't much
fancy dismantling CI pipework off a ladder, given its weight and the
likely need for 'destructive dismantling'.