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I have filled the grid, apart from the unchecked letters of the encoded entries. I know what those entries are, have identified the source material and located the three required highlights. I have written out all the pairs of letters and their encoded equivalents that I know but I still can't see a way to find the key phrase. I even tried a playfair solver online but that made my brain hurt. Can anyone point me in the right direction?

An online solver won’t be much help, because the encoded words are proper names and not in their dictionaries. The way in with Playfairs is usually to find a couplet whose encoded form shares a letter, eg AX > XB That tells you that the letters AXB appear in the same row or column, either in order or wrapping round at the end and start. So it could be AXB . . , . AXB . , . . AXB, B . . AX or XB . . A (in a row or column). If you know the theme you might find a promising sequence that way

From writing out the encodings you should have identified HE > ER, HF > CH, LO > OH, ND > DS. They would be a good starting point. Remember, they can wrap around in the same line. AX > XB implies AXB, and XA > BX also implies AXB

Goodbye JEG. Thank you.

Thank you Dodgepot. I think I have it now.

My heart sank when I saw it was a playfair puzzle. I have never really got to grips with them and resorted to breakers in the past which as others have said don’t work here.
So I did some research and learn how to crack a playfair.
A really rewarding process. I might even look forward to them in future.
What a good end to the Listener year.
PS I found this site really useful. https://www.oranlooney.com/post/playfair/

Goodbye JEG - thank you and Best Wishes.

This was rubbish until we tried the 2, 12 and got lucky.

Then it was absolutely brilliant.

Plus thematic freebies including the nemesis in connected cells.

Thoroughly enjoying this stern final test from Dysart. Completely stumped by both 2D and 3D however - any gentle nudges to how they might break down would be most appreciated!

2d - An odd name but the wordplay is pretty straightforward (2 letters + 1 letter + French 'senior').
3d - 3 letters for 'elfin' around musical 'restrained' (abbrev).