Remember that ITV Gameshow documentry that aired a few years back- it had some broadcast quality clips of TVS Catchphrase in it, so someone must know where the tapes are and was able to get them to use in that programme.
I can guess the route from archive to screen with that one.
Catchphrase has a lot of connections to Action Time.
One deeply suspects that all editions of Catchphrase (even the TVS ones) were brought by Action Time, and then Action Time was brought by Carlton - so, therefore now sitting around in the ITV Archive in Leeds.
It most definitely was not purchased outright by Action Time, but it's entirely possible they had clean, high quality copies.