The leak in question is on a Dan Wesson 715 recently bought used. It shoots fine and there are no audible leaks. The co2 capsule seal looks to be in good condition. If I leave a partially used capsule in its empty the next day.
Cheapest atf stop leak I've seen is £6 so I was thinking it's maybe worth a punt as a reseal is a faff and likely to cost more.
If it doesn't work then I can always use the fluid on the car!