Thank you to those above for the hints. I've nearly finished but still have a few to get, including the F despite the hints. They are mainly in the northeast corner according to the way I've filled in the grid.
I still have to get the X and the and the F in the 8 letter clues and the I,, the P and the W in the 6 letter clues. The P and the W have to end in the same letter.
I addition there are several I can't fully parse - the A, the G
Just had a penny drop moment and got the F.