Tom wrote:
Most manufacturers seem to recommend setting flow rates in CH systems to
achieve a rise of 11deg across boiler, why is this? is it the most efficient
setting for heat transfer?

It's a compromise.
For condensing boilers, you want the exhaust output temperature to be as low
as possible, otherwise the efficiency will drop as it won't condense properly.

On the other hand, you've got to get temperature high enough to heat
radiators and central heating.
So, the smaller the temperature rise, the higher the efficiency.
Even for non-condensing boilers, for non-pressurised systems, the
boiler can't get past some 90C or so, otherwise you get localised
boiling, which does all sorts of nasty things.

Both of these mean that you want the temperature drop across the
boiler to be relatively small.