By today's standards the Spectator puzzle is unfair, in particular in terms of the proportion of unchecked letters in some words and the use of complex wordplays involving obscure words to indicate entries which are themselves obscure (I knew less than a dozen of the words to be entered).
However, we must remember that the puzzle was set nearly 50 years ago when setters of barred puzzles typically aimed to challenge solvers up to (and often beyond) their limits, rather than to 'do battle with the solver and lose graciously'; today's focus is more on the latter, and on providing entertainment, which was in short supply in 'Indefinitive' (considering the clues were 'wordplay only', they were extraordinarily poor by today's standards). As recently as 1980 the Listener puzzle Lip Service by Leon received no correct entries; today's editors would not publish a puzzle that they did not expect a good proportion of solvers to be able to complete. J. A. Caesar (Jac) set 52 Listener puzzles between 1958 and 1975, and I suspect his early Spectator puzzles were pitched at a similar level of difficulty.
I believe that I have parsings for all the solutions (assuming I've got them right!), so if there are any that you would like to clarify just let me know. The clue for 36dn ("Shout 'eight' perhaps") was probably the most outrageous of the lot.