Thanks gents. I think you are right. I have been working from an old Honeywell wiring diagram that doesn't have that wire because it is a diagram using a Randall 3060 which did not have an option of CH without HW. The wire you mentioned is missing.
If putting the wire in doesn't fix it I...
System:
Glow worm Hideaway Boiler (fairly ancient)
3 way valve Honeywell V4073A (Brand new)
Controller TH-0471 (No temp control) (Brand new)
Roomstat Danfoss TP5 (2yrs old)
Cylinder stat Honeywell L641A (Brand new)
My previous controller was a Randall 3060 which I have just replaced so...