If your gravity HW system heats the water fast enough, your best
solution -
rather than going fully pumped - is to convert it to a C-Plan. [See
http://content.honeywell.com/uk/homes/systems.htm ]

For this you will need a 2-port zone valve and a tank stat on the hot
water
cylinder (if you don't already have one). This gives truly independent
control over CH and HW, and also provides a boiler interlock so that the
boiler only fires when either or both circuits require heat - and shuts
down
when both demands are satisfied (i.e. when both stats are open) thus
saving
fuel.

Looks like order a "2 port valve then"

An I right in thinking that to wire I need to:

Connect thermostat HW On to: Valve Brown
Permanent Live to: Valve Grey
thermostat CH On: Valve White and Pump L

Boiler L to Valve Orange

plus neutral & earth


Thus, boiler gets power when valve activated (via grey)
Boiler gets power when valve off (via valve white)

Currently the boiler has a connection for the pump. I assume that this
should be left so that the pump can continue to run when the tank
theremostat is cycling.