ive never had a problem with the ones they do in plumbcentre. you can get a bag of 10 for about £20. They are decent quality, cant remember what make but decent brand. Personally I rarely fit them, and more often than not take them out. I work by a rule that if there's at least 2 places to isolate a circuit upstream then thats sufficient for maintenance. These valves never get used then when they're needed 15yrs later they leak, even the good ones.