But surely, if you do this without additional relays, you have to power
the
boiler and pump from the same feed - effectively connecting them together.
When (in the summer, say) you turn the boiler on to get DWH, the pump will
come on too.

Well, one relay will be required, now you come to think of it, although no
doubt someone will come up with a clever relay-less design if we wait long
enough, using the NC terminals of the cylinder stat.

Christian.