It's no problem fitting the valve in the CH return as there's plenty of room outsite the boiler. The DHW might be interesting.
The pump plan is sold as having the benefit of getting rid of electric valves which could suffer failed motor, sticking mechanism, etc. They are not half such a maintenance problem as two tiny valves which are sensitive to inevitable crud.
With the flap valve fix I can use a desludger, drain, add valves and a Magnaclean and refill. The rads aren't blocked, so there's not much excuse for a powerflush, except on the DHW circuit to clean out the boiler.
Thanks for your help.