The best way is to fit a 3-port "diverter" valve to give a priority
system.
A diverter is similar to a 2-port valve with just another port. The boiler
will need to be on full temperature. A simple clamp-on pipe stat on the
flow
can be fitted to keep the CH flow temperature down if that is a problem.
Better still, if the cylinder is old replace it with a quick recovery
cylinder to reduce time heating the cylinder. Travis Perkins do the
Telford
Typhoon very reasonably. Doing this will transform the system for little
outlay.

IMM,
I prefer to have valves that allow heating water and rads simultaneously if
needed, so I would not be happy to use the "diverter" W-Plan option.
Looks like it is going out of favour now also,as I did not see it as an
option from some manufacturers and found an article recommending them to be
replaced where found see -
http://www.gasman.fsbusiness.co.uk/f...ed_systems.htm

Thanks for your suggestion on quick recovery cylinder - I want to get one,
but so far only found the "Superduty" by Albion.
And thanks for your recommendation of good valve makes.

Dave