Lumen, yes, extra letter in lax. If you use the crossword tool on this site with the letters you have there are very few options for 1d. Then I would focus on 22a, 24a, 29a and 33a.
4d should be clear from 1a now.
There are NO options for 1d on this site’s quick solve, for the letters I have?
(I have 14a beginning with an S, and the first three letters which I’m pretty sure are right).
I just tried quick solve with the 1st, 3rd and 5th letters and it produces one possibilty (the correct one). What do you have for the 3rd letter? You are right re 14a.
I'm not very far advanced, but the word with the 1st, 3rd and 5th letters I have lead to only one answer on this site's quick solve - but it has E as the penultimate letter (14a) not S?
Sorry if I put Lumen on the wrong track! Regarding 20d I guessed what the first letter of 22a was from the clue and only fully solved it later. Similarly 35a having spent time in the 70s doing grainy black and white photos on the south bank of the Tyne around the abandoned wharfs the (untreated) term was a familiar one (:o)