Heating Mystery

[namsag]

A Properly wired frost stat will not bring on the heating or hw it is for boiler protection not system protection.
Why are people presuming 3 port valve OP doesnt say

 

[Greg12]

It's a Tower mid position valve.

Don't think it can be the frost stat because it is only running when the hot water is switched on.

Much appreciate all the thinking going on on my behalf, guys

 

[namsag]

Other than valve problem, letting water pass when just on HW.
Your system may have a return in the wrong place.
From cold turn just HW on and as rads start to warm up see what pipe heats first then stick heating on and see if it changes to the other pipe.