I was at Surestop a coupe of months ago. Got to see them, strip them (the valves) and rebuild them. It's a really good bit of kit and the quality control is tight too. Grit or dirt is the main issue, but the valve is easy to strip and sort if necessary. The gang have a tech helpline if you need advice. I understand that British Gas may be offering to fit these (if they haven't already started), so if you are thinking about offering fitting as a standalone service, there will be competition. Would I recommend it? YES no question. I have also fitted these instead of usual service valves for washing machine supplies.