One heating and one HW. Definetely sounds like a wiring problem, just asked my dad about it and he says its always been like that, ie pump overunning, and heating coming on when calling for HW only.
Just to confirm, you have two motorized valves - one in the CH circuit, the other in the HW circuit.
The main power connection for the system should be to the boiler, which then provides power to the pump, motorized valve etc. It is important that the wiring is as shown in fig 30 of the manual.
The conventional wiring of an S-Plan is shown below. But the power to the junction box comes from the boiler. The wire marked DV (Demand or switched live) in the boiler manual and the pump all connect to the same terminal. This is equivalent to the wire from terminal 10 to boiler terminal L on this diagram.
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