The story I heard - and, again, this came from a source very close to the Shads - was that Bruce didn't want to record 'Argentina' and made this very clear by launching his guitar across the studio and walking out! Fair enough, we all have our off days and I have witnessed such things myself when playing in bands. Indeed, on one occasion when we were rehearsing a song that I thought was rubbish, I just got off the drum stool, lay down on the floor and said "I'm not playing this."
The thing that really surprises me about 'Argentina' (as I alluded to earlier), is that although they were trying to make it sound like the 'live' version, which it did at first, it ended up sounding far less like the live version once it had been shortened and the electric guitar replaced by the acoustic. I will admit that the acoustic guitar does sound nicer though - warmer. Brian did a great job, however, and it was many, many years before I realised that it wasn't really 'live' at all.
Cavatina was a great follow-up and I think this came about because Bruce saw The Deer Hunter at the cinema and recommended the tune to Brian and Hank. So glad that they made a video for it, with Dave Lawson and George Ford.
https://www.youtube.com/watch?v=TUBsQVVfrYoand TOTP 1979
https://www.youtube.com/watch?v=ERRDkUliStw