Huw,
It depends on the individual, some might feel that an extra hour or attending the third class may confuse them because of the introduction to advanced Kata. Others I believe would get a lot out of it just by performing oi-zuki a couple more times, just to ingrain a feeling or something.
I think that if possible (work commitments, bringing kids to train etc), anyone over 7th Kyu
can attend these sessions.
I totally agree with you Huw, that
EVERY 1st Kyu should attend these sessions on a
REGULAR basis (not two or three before the grading).
Other aspects of the third class are things like, Hente, Haito, Go-no sen and things like Kata bunkai for Bassiai and above.
A lot of the combinations used at these classes are not strictly from the U.K.T.K.F grading programme, rather, they are from the notes of the instructor who has trained with such notables as Sensei's Kase, Ochi, Osaka, Aragane etc, etc (it's a good game to try and guess who the combination \technique etc belongs to, tip, Eryl always uses Osaka Sensei).
There we have it, the myth of the third class explained (well, some of it

).
Mole Bach