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Apologies for the second O, which was an aberration - the last five should be D (fish not dish in 20D), O (rats not rots in 22D), F (tree not free in 23 D), R (pack not park in 25D) and M (run not rum in 28D).
I still have no idea what to do next - somebody in the Answerbank has said that 'As far as the theme and the 2 extra letters, there is really only one "elsewhere" where they can be applied to "clarify the theme".' Oh really - I have been staring at this for three days now and I have absolutely no idea what is required!

From your list of 12 letters MLMTPTADOFRM,(these are definitely right), two (which are the same as each other) will be left over when you've done your changes to the entries in the grid. They are used to alter the only other major possibility as a thematic indicator remaining to you. Look in the grid for words which, with one letter alteration, might form a generic group. How about 17 & 27 across for instance? Bradford's is very useful.

P.S.

it's only the altered words which are symmetrically situated, NOT the altered cells.

If you apply the 2 extra letters (T's) then
Men Are Fools becomes Ten Are Tools

The discarded letters must be used to make implements (tools)

turast,many thanks for help completed at long last

You are welcome, Leon. I tried to give strong & useful hints without completely giving the game away. I don't understand why Tony did that.