I am a very recent recruit to 1across, so apologies for the relatively late posting. I picked your puzzle at random out of the section, and can honestly say that I enjoyed (nearly) completing it yesterday. Up until now, my experience has been almost entirely with the setters from The Independent and i newspapers, and your puzzle seemed to fit that style and degree of difficulty very well.
I have since read the various feedback comments, and to some extent they match my own views. From the point of view of a relative newbie, who is keen to learn himself, may I make the following observations?
11 I liked this, but from much of what I have read, should not the definition be separate from the wordplay? If so, I would probably add “Sin,” at the start, which I accept would make it a good deal easier.
16 To avoid the repetition of “South”, I wondered if there was some way of defining “South Kensington” by the presence of the various museums there, which should be fairly well-known? The “south-east” could then be simply “SE”.
24 No, I didn’t know the Cambridge reference, but I am quite used to Googling various classical and literary figures, so I don’t see that as a problem in itself. In fact, if you were going to stick with it, I might simply go with “Director’s chair?”.
6 A very minor quibble, but to most non-Londoners, I suspect the Barbican may be known as an arts venue, but less so for the estate.
15 I didn’t get this, but I will have to put that down to my wordfinder not listing it – the clue is fine!
17 A neat clue, but I am slightly uncomfortable with the “Where”. Personally I would use “How far to lower one’s standards after tragedy?”
21 Would “but with nothing for head” make it more obvious that a substitution of the first letter is required?
22 I got the Roman numeral part, I just didn’t initially find a definition of calx that made it clear that it was a remainder.
A very enjoyable puzzle on the whole, with some very inventive surface readings, and I learned a few new things to boot. Many thanks!