I think the limitation must be the simple 3-port valve which has to be one or tother and not both.
This is what I've found out (or I think I've found out) for this installation:
When power switch is on, power goes permanently to boiler, COM of HW stat and COM of room stat.
When HW stat calls for heat, power goes to switched boiler feed, then on to pump from boiler and power is off to the 3-port valve.
When HW no longer calls for heat, then power goes to the 3-port valve so it is always powered and switched over for CH when HW does not call for heat, regardless of whether room stat calls for heat.
When room stat calls for heat, it switches power to switched boiler feed and on to the pump. The normally-on connection from room stat is not used. But at this point if HW is also calling for heat then power to diverter valve is removed so hot water goes only to HW.
This is one of the pieces of information from
http://www.diyfaq.org.uk/plumbing/controls/controls.html
"W-plan systems can leave you without CH for some time since they can provide heat to only one of HW or CH at any given moment. Usually HW is given priority, so if you run a bath all heat will be diverted to the HW for as long as it takes to reheat the water. If several people have fresh baths or showers (particularly power showers) in succession you may be without CH for an hour or more. This effect can be reduced by fitting a fast recovery cylinder (which has a more efficient heat exchanger and therefore heats up much quicker). People have been known to be so dissatisfied with a W-plan installation that they have had it converted to Y-plan."
(Mick, As you suggest, there is no timer on HW and the room stat is purely CH.)
I really appreciate your efforts to help me sort this out.
Thanks,
Francis