Radiators still slightly oneven though Hot water is selected

[ghost32]

Hi I live in a 1970's council built property, and have gas central heating, (back boiler behind a gas fire) HW/CH electrical timer on the wall, a hercal indirect coil cylinder upstairs, myson compact pump (well that what the name says and I believe some kind of electrical directer switch, mid position actuator code BGMVSP-23 (a blue box that can be take of the pipes that has a turning mechanism on i that turns something in the pipe junction. (you can tell i know alot about this)

My issue I have is that even though I have turned off the central heating and only had the hot water on timer, the radiators still seem to get warm (I haven't checked for sure but i'm guessing it's when the hot water is being heated.)
I'm guessing what ever divert valve/switch that controls the flow between each, or the valve that controls the flow to the CH is not fully closed/worn away. can anyone give me some advice on where to start looking or anything I can test for to confirm this? and if poss, what needs to be fixed and can I fix this?
Many Many thanks
Mark

 

[ChrisR]

Your guesses are all correct.
It's called a diverter valve, and yours is "letting by".
Replace it with a honeywell one. Means draining the system down, but the nuts and wires will fit and they give less bother.