Dave wrote:

To estimate the impact of changing pipe bores I'm planning on using an
ohm's law analogy and "series resistance". Can I just say that the
relative "resistance" caused by 22mm and 15mm pipe is directly related
to XSA (a factor of approx 2) of are the surface effects significant?
It's a long time since I did any fluid mechanics, any Bernoulli experts
out there? ;-)

IANA fluid dynamicist but ohms law for pipes is a bit different to the
electrical one. For a given 'resistance' (pipe type, size & length)
flow is proportional to the square root of the pressure drop, or 'head
loss' as it tends to be known. Or, put the other way round, head loss
is proportional to the square of the required flow rate. For a given
head loss per unit pipe length, flow seems to be roughly proportional to
pipe area.

http://www.ukcopperboard.co.uk/downl...-%20basics.pdf
has a pipe sizing chart (nomogram) which you might find useful.

--
Andy