Thanks to all for the helpful comments, I knew the collective brainpower would sort me out (and I was right to be worried about posting my incompetence).
To explain how I got into this position, I decided to go for W plan as I have a rapid recovery cylinder and an old house with high thermal inertia: so losing radiator heating while recovering the cylinder on occasions of heavy hot water demand didn't seem like a problem. Also the diverter valve seemed a little simpler and perhaps more reliable than a mid-position.
I went for a Vaillant boiler with their fancy electronic 140 timeswitch which needs the VR 65 control unit. The timeswitch tells this what to do via a low voltage link (described as a bus, so presumably some sort of digital link). I hadn't been able to find the datasheet for that on the web at the planning stage, so I didn't realise until the hardware arrived that it notionally only supports Y-plan (mid position) or S-plan (zone valve) systems, and not W plan. But by then I had a diverter rather than mid-position valve plumbed in as described with B (closed on power) to the cylinder and A (open on power) to the radiators.
But I thought never mind, I can ignore the timer-controlled supply which opens the CH side and use the S plan timer controlled DHW supply to actuate the diverter valve. That was when I thought the diverter valve OPENED to B on power.
In the summer, I'll have a timer program to provide DHW only so that on a cylinder thermostat demand, the boiler will power up and the diverter valve will operate so I'm only heating the cylinder. In the winter, I'll have a different program calling for CH and DHW so timed CH demand will heat the rads except when there is a DHW demand as well, at which time diverter valve will operate until the demand is satisfied, in other words W plan logic.
I have TRVs throughout except for a towel rail, but the new boiler has an automatic bypass and fancy protection so I guess I could put that on TRV as well.
Or am I still really missing something? For the case where I have a cold house and a cold cylinder I suppose I could have another program for CH only although, to be honest, that's probably where I would fire up the woodburner and be wanting hot water ASAP.