I don't know if this info will be any use to anyone, but the new cast
iron that my son is putting up is approx 70mm inside dia and 80mm
outside. The other end has the mounting and larger dia for the next
section to fit into.

I meant to take a micrometer, but forgot so just measured with a rule.

He has 2 of these pipes that take water from a large roof in the next
garden down a sandstone wall into a soakaway.

His brother and a friend came to help him last weekend. They spent 4
hours setting up the scaffolding tower and inserting wooden plugs into
the soft sandstone, while he was "unfortunately" forced to take a long
and important business call. His brother was not very pleased. The pipes
are still lying on the ground, which is why I was able to measure.
--
Bill