Boiler overrun contacts.
Boiler overrun live goes to NC contact.
Heating Demand Live geos to NO contact.
HW Pump Live goes to Common.

Boiler control Contacts.
Heating Demand Live geos to NC contact.
Unswitched Live goes to the NO Contact.
Boiler switched Live goes to common.

Someone may be able to do an ASCII art version of this, It's beyond me.

Yes, I think that works!

Inputs to system:
CH On = Central heating on (at programmer)
HW On = Hot water on (at programmer)
POR = Pump output (from boiler)
N = Neutral
L = Permanent Live

Output from system:
BCFH = boiler call for heat

Pumps:
CHP = central heating pump
HWP = hot water pump

Switchgear:
rts = room thermostat
cts = cylinder thermostat
NO = normally open terminal
NC = normally closed terminal
C = common terminal
/\/\ = relay coil

/
CH On O---o o-------+--CHP---+---O N
rts | |
+--/\/\--+