Several years ago, I asked here for any ideas about why one of my bedroom radiators heated up even when the house CH was not On, only the DHW. The general response was that there is a certain amount of autosyphon heating circulation taking place in a 2-storey heating system. The system is a Plan 'C'.
I think I have the answer. During the work detailed in the thread 'DIY with Honeywell Thermostat', I found that there were no wires connected to Wiring Centre terminal 8, but to terminal 9 instead. This means that whenever the tank 'stat tells the zone valve to open in order to heat the tank, the CH pump is also activated, thus causing circulation for a short time. Just enough to heat the radiator nearest to the storage tank.
Since we (now just I) have used this to our advantage by helping to heat up the bedroom even when the downstairs room 'stat is not asking for CH, I will leave it as it is, at least for the winter. I may think about switching the wires back and forth for Summer and Winter settings.
QED.