I've installed various undersink water heaters, mainly 3kw=15litres. They come with 2 plastic(or similar) connectors which connect the 2 1/2'' fittings on heater to the flexy's im using. The destructions say they are DIELECTRIC JUNCTIONS? What is the reason for these? I,ve bought a couple of heaters of e-bay and the dielectric junc. are missing. Who sells them? Part cen? Hargrieves? screwfix? or toolstation?