Hi, can anyone offer any advice as to why the heating would run when HW is called for?

[davesparks]

If you buy a number of electrical components and integrate them into an assembly or enclosure you are then classed as the manufacture of that assembly.

Irrespective of if the commoners used are CE marked the assembly as a whole must go through a product safety/conformance process.

We are talking about a purpose made wiring centre, with relay fitted in it, that is supplied as part of the pump plan kit. The installer doesn't assemble it from components, they connect the cables from the various controls to the labelled terminals.

The manufacturer CE marks it, the installer installs it.
If the installer has to CE mark it then they have to CE mark every damn joint box, socket, light switch, light fitting or anything else they fit.