water AND heating when I want only HW

Although I have set my Landis & Gyr control box to produce only Hot water ...the Heating comes on as well anyway !.

In addition at various times of the day , I get a small overflow of water through the overflow pipe to the outside of the house ...

Can anyone suggest why ...and if these problems are related ... I'm confused ...

One possible cause of both of these things is a dirty system.

That is to say, a closed circuit to which insufficient chemical inhibitor has been added, leading to corrosion of the steel parts, forming magnetite and particles of rust that can block valves and pipework.

That's my suggestion based on the information you've given. Maybe now you can open the box and reveal the star prize of the full details of your system?