Thank you so much, mattrom, for taking the time to respond to my "belated" request for help. I was concerned that the thread might have been "closed" given everybody contributing to it seemed to have had all their queries resolved.
It's funny how the brain works. Two minutes after posting my cri de coeur last night, I got the answer to 31d and kicked myself for having been so obtuse, as it was probably one of the easiest clues in the whole puzzle.
That left 42a and 41a.
I had already solved the thematic clue 36d, and obeyed the thematic advice of amending its first two letters to accommodate both 35a and 38a. I had naively overlooked the possibility that its 4th letter might also need to be changed to accommodate 42a, which is why I got stuck before receiving your help.
That left me with 41a, for which I already had the 1st, 3rd, 4th, 5th and 6th letters from the 5 intersecting down clues. With the other 3 thematic words, I was "fortunate" to have solved the clues (and written in the answers) prior to then having to amend various letters because of non-themed intersecting answers.
With 41a, this situation was reversed. You gave me the help I needed, and I thought I was home and dry. But there was a problem. All the other thematic clues had some letters amended to accommodate the answers to non thematic clues but, in the case of 41a, it also intersects with another thematic clue (25d). How, therefore, is one to decide what the 4th letter of 41a should be? It can either be T (the unchanged last letter of 25d) or N (the as yet unchanged 4th letter of 41a).
I would hate to have an incorrect solution caused by a single incorrect letter. But it does seem that the "advice" of how to treat the answers of thematic clues produces two possibilities, depending on whether one has solved 25d before 41a or vice versa - and that surely can't be right.
Two final questions :- 1) What is the relevance of the puzzle's title "Potatoes"? and 2) Is my pedantry justified in saying the word "into" in 41a is not correct and should be "by" or"beside" or "after" etc.
Sorry to have banged on for so long. I really would appreciate a response to my problem over the T or the N.