The first step was to add some additional sockets and a dedicated spur for the central heating.
Checking the load on the 16A supply:
Shower pump – 500w
Central heating pump/boiler – less than 100w
Network switch, other adaptors – 120w
Immersion heater – 3,000w
Total: 3,720w = 15.5A – so the supply is sufficient, and it would be rare that all devices are on full load at any one time.
Today I added an additional double socket and fused spur, and re-configured the layout.
An Idratek QRH Din module fitted perfectly in a Gewiss GW40102 enclosure. The new immersion heater cable goes into the enclosure from the fused spur, only the live cable is cut and connects to the relay contacts (output 4), and then feeds the immersion heater.
The pink cable (mains rated) connects the module to the Idratek network.
I have also added a five-core cable to relay outputs 1 and 2 to control the central heating (hot water and heating), and a two-core cable to one of the inputs to sense when the hot water pipe is getting hot (and therefore the boiler is working).
The finished enclosure looks tidy: