After pestering Grundfoss, Wilo and Bosch (who incidently insist on an automatic bypass but are unable to advise how it should be set up) I got a very sensible reply from Grundfoss and installed a Taconova setter valve. I t works brilliantly when the boiler shuts down both motorized valves close and the bypass opens up to the desired bypass flow and not to waste the heat I put it back into the hot water tank.