I've sorted the answer to my own problem (as below)...a new diverter valve spindle kit is required!

Help! Heat Line problem?
What is causing poor hot water flow to all taps? So much so, that when one tap is turned on (with only more than a trickle), the other/s goes off. All three taps in the property are mixers, and I've tried relieving them of air...not easy.

The boiler fires up each time when calling for water, but flow is extremely poor.

The central heating is OK, but there is also a leak from the diverter valve onto the diverter valve motor.

RSD, February 2009