As far as I am aware the R6660D is the receiver for a wireless wall mounted room thermostat, I seem to remember a wireless tank thermostat, seem to remember (CS92) was the number, but when I tried to get one, they seem to have dried up, so maybe same receiver used for both?
But need a bit more info, boiler type, installation plan, and even if water is referring to domestic hot water.
Also Hive single channel is volt free but hive duel channel is 230 volt control only, and the unit you show is clearly using volt free connections.