Finished, after reading the forum, where a couple of tiny nudges worked wonders: roof's mention of heads set me on the way to12, even before the more explicit help in a later clue.
I struggled with 13/8 because first I guessed "engineering" for 8, as an 11-letter business process! Then twigged the first part of 13 but thought of "blind" for next part.
Foi: 11a, but then rubbed it out on finding 21a. Had it wrong, ie had written the 21a answer.
Loi: 2d: the whole N w corner came last .. ah no, 17d, 25d were actually the last pair.
At one point I thought it was exasperatingly difficult, but the cleverness of it outweighs that ( now that I've finished). Rather a splendid puzzle, thanks Brendan.
Is this Brendan the same Brendan who is a regular here? I hadn't spotted the matching name!