I cannot wire the CH On output to Swl as it would work backwards - i.e HW
On
we would then get pump.

There is usually a zone valve with an additional switch that fires up the
boiler in parallel with the hot water system.

Alternatively, a relay may be used.

How can this be rewired to give CH Only. I reaslise that there is
currently
no valve on the system, so HW temp control is only provided when in HW
only
mode. If CH is running then there is no control of HW temp.

This depends on your boiler. Some boilers require an open loop through the
hot water cylinder to dump heat. Some require careful routing of pipework to
ensure emergency venting and quenching water supply. Some don't care at all,
enabling you to place zone valves on the hot water circuit. What is possible
depends on the boiler design.

Christian.