Hi Jo,
I agree about the probable anagram at the 3d but have tried it in every anagram solver I can find, all to no avail.
12a I too thought PET is contained but as the first 3 letters of the first word, making it PETE, which I know sounds unlikely but there is another name in answer to 5a:-
What Dr Spooner did with his flag (4,4) - and the answer, again by Ratty and his Mum is DAVE WITT
This, coupled with 20a:-
Eg 5A, 12 etc. held back in carrot I'd eaten (6) - Answered as EDITOR by Ratty and Mum
I should mention, also, that the R and E in the 2nd word of 12a are seem correct. They were both answered by Ratty and Mum and the R comes from:-
23a Timely chip in Powerpoint extension (6) - PROMPT
and the E from:-
25a Deaf? Stay away for this anniversary (5,3) - FEAST DAY
Anyway, I think we've probably both spent enough time on this. and who knows, we might even hear from the aforementioned Ratty and Mum at some point!?