nospam writes:

Is the 22mm overkill? After all, don't some rads use even smaller
pipes than 15mm? I bought a 15mm rad valve recently (tool station)
and was surprised that the bore in the valve was so small: less than
5mm I would guess. So wouldn't running 22 to it would be redundant
because the bottle neck is at the valve? OTOH I would think it
better to be overspecified and have reserve in your system rather
than run it to its limit.

Tubes which are too big have at least one downside: having to wait a
longer than necessary time for the hot water to make its way to the
most distant radiator(s) when the heating starts.

URL:http://www.cda.org.uk/megab2/build/pub-150-copper-tubes-in-domestic-heating-systems.pdf
gives more detail about optimising the diameter of each part of the
system than the `guidelines' the OP asked for, but IMO it's well worth
doing.

--
Mark