The message
from "george (dicegeorge)" contains these words:


reading between the lines i guess that the old black pipe is
polyethylene which i should replace,
and that the blue pipe is UV proof,

Is it ok to buy blue pipe for the cold rising main
from the farm supplies shop down the road,
and what width of pipe should I get?


There are various varieties of "old black pipe" to various standards,
but for your purposes it's enough to assume that the old pipe is to
imperial standards and will probably be OK long after you're gone.

The blue pipe will be to current dimensional specification but is not
designed to be UV proof. However, it's not going to disintegrate if the
odd bit of it is visible inside out of direct sunlight.