Question Author
Ah, I found the relevant paragraph:
"To comply with water bylaws, the height of the shower should be such that when the sprayhead is hanging loose on the flexible hose, the sprayhose does not drop below the spill over level of the bath or shower tray which could lead to contamination. However, there may be installations when this requirement cannot be met. If the sprayhead can be placed within a bath, basin or shower tray, then a double check valve, or similar device, must be fitted in the supply pipework to prevent back-flow."
But I still don't really understand. I mean, even if you dipped the sprayhead into the bath water, is the bath water really going to rise up the hose and into the public water supply?