Simple job? Don't be silly. Long story short, blocked downpipe [1],
but now need new downpipe. The bottom half is original cast iron, and a
short test length of modern plastic 68mm pipe is a bit loose, say 2 or
3mm. Will that matter? Fall is vertical, and I would prefer to retain
the bottom six feet, which is cast, and just replace the length from
there up to the gutter.

[1] Victorian house, mostly still original cast gutters and other
fittings. Discovered today that one downpipe is original cast iron for
the final six feet, but the eight feet or so above is something I have
never seen before. There is a seam at the rear, and the material is
metal, possibly galvanised, zinc or similar. Quite thin - trying to
remove it, I put my thumb through it.
--
Graeme