At each valve 21, 22 . . . 2n, a respective decoder 81, 82 . . . 8n is provided. Live and neutral lines 10,11 pass from the decoders to the valves. Each decoder 8n has an input connector 21 for the live and neutral pair 6,7 powering it and an output connector 22 for the further live and neutral pair 10,11 to the respective valve 2n.