Those Lewton titles were each broadcast by the BBC one-time only, in 1982 and 1993 respectively. I recorded both and recall Mademoiselle Fifi was notably soft (16mm source?) and televised under the alternate UK (?) title The Silent Bell, also in the Radio Times listing. I'm not sure if I still have the tape as I replaced it with a French DVD which is not much better but uses the original title. I recently caught up with Christian-Jaque's 1945 Boule de suif which, at around 105 mins (much longer than the imdb running time), for me plays like a bloated remake of Lewton's more compact film, following his lead of combining two Maupassant stories.
All I know about the BBC's allegedly "leaky" film library roof was quoted in the New Scientist article, of which I must have a cutting somewhere. I do recall that at some point - late 1990s? - the BBC seemed to switch to video masters of the RKO films which looked as if they'd been digitally cleaned-up but also often had a softer, smearier, greyer appearance (perhaps suggesting standards conversion in some cases) than the rougher but sharper physical prints which had previously been televised. Of course, the difference wasn't as noticeable on small CRT sets, especially when played back on domestic VHS tapes. But it was also around this point that the range of RKO films aired by the BBC contracted sharply; many previously broadcast titles I'd taped in the '80s were no longer shown but several dozen of the most popular were endlessly repeated (at least until the surprise season in 2013). This may have been due to technical issues or a decision based on ratings. Around 2000 Channel 4 also stopped showing the more obscure pre-1950s films.
I remember this "video look" when the complete version of The Body Snatcher was broadcast compared to the heavily BBFC-censored copy which had played in the 1980s. I wondered if the BBC obtained the uncensored version from the US rightsholder. There was of course also the exciting 1994 Lost and Found season, during which The Ghost Ship finally turned up on the BBC. I don't know if lack of good materials or the famous plagiarism case against RKO had prevented a previous broadcast; it had definitely been available for purchase (complete) on Super 8 since the 1970s - I bought a print - and I think 16mm rental too, though Joel E. Siegel called it "virtually a lost film" in his 1972 book on Lewton.
I don't know about RKO titles but my understanding is that when film prints were broadcast 16mm was often used, as suggested here by a friend of Philip Jenkinson recalling "Warner Brothers offered him their entire UK-housed stockpile of 16mm prints after their TV license deal expired rather than pay to ship them back to Burbank".
Some of the above is mere speculation and memory by an interested spectator and I'd be delighted for my theories to be corrected - or confirmed - by the professionals involved! (I've no idea what's used today by the BBC for their RKO titles as I haven't had a licence for several years.)