Are you running the radiators from the store as well, Christian?

No.

Otherwise, if it's a condensing boiler, does it have separate
temperatures of operation when in DHW mode vs. CH?

The boiler is a Worcester-Bosch Greenstar HE System. Does this have the
ability to have call for heat signals at different nominal flow
temperatures?

The system will be a Honeywell 'S' plan. Initially, it will have 3 zones,
upstairs, downstairs and water heating. This will ultimately be expanded to
5 zones, for the loft conversion and conservatory. As planned, there is no
hot water priority. Although I could possibly contrive such a system, it
might require relays or similar to disable the heating circuits when the
water valve is open. I'd prefer to avoid this, unless it will significantly
improve performance.

Then you have to think about whether the radiator sizes are adequate
at the reduced temperature to give the heat output you need.

Probably. I can turn the blending valve up in cold weather. If I install a
blending valve, do I need an additional pump on the heating circuits?

Christian.