[James2001]

I'd be fascinated to see this episode of Casualty with the subtitles complained about. Something that sounded like "effing" must have been said, twice. I wonder what it was.

A quick look at an online TV guide (tv24.co.uk  ) suggests it was this episode, the final one of the first series:

uktvplay.co.uk 

I presume the offending subtitles have been removed, but if anyone wants to watch the episode to work out what could possibly have been mistaken for the word in question...

A look at Genome does say that unlike the rest of the series, that particular episode did originally go out post-watershed, at 21:25 , but I still don't imagine it had that word in it. Especially as it immediately followed the Paul Daniels Christmas show which would have had a large younger audience.

genome.ch.bbc.co.uk 

The DVD release that came out in 2006 has no strong language warning, only a mild one, and the episode in question doesn't even have the mild language warnings that several other episodes have (and it likely wouldn't have got a 12 rating if there had been 2 f bombs in the space of 50 minutes):
www.bbfc.co.uk 

Funnily enough there's been plenty of other, not quite as strong words left in Drama's showings of both Casualty and The Bill that in previous years would have been excised from daytime showings, I presume there's less squeamishness broadcasting some words during daytime these days.