I want to do some alterations to my outside garden tap but cannot locate the isolating valve. The tap is mounted on a wall about 6ft from the house and 2ft above ground level. The supply runs vertically downwards from the tap and is routed underground to the house under a concrete path. The supply pipe is 22mm OD and looks like a heavy duty black rubbery plastic. I’ve hunted behind the kitchen sink unit around the house main stop tap but cannot find it. It’s been like this for over 40 years since before we moved in but the situation is worrying. Any ideas would be welcome.