I’ve found on certain things that come from manufacture, aren’t imo how they should be. For example, had intergas boilers come with the drain off not fully closed, that type of thing. Perhaps this was the case of your valve?
It leaked from the drain off point and the oring wasn't sealing properly.
Just looked a poor design.
It wasnt a screwfix item, the ones they sell are 3/4 union with a rubber
Are you on about the 3/4” type that Scott referenced in photo initially? A quick google search throws up a few places, inc BES and City Plumbing. I knew them as wasp valves, but they probably offended wasps, so maybe a name change 😂😂😂