I have three quarters of the grid completed, with the bottom right part causing problems. I know what how to enter the relevant answers in the top left quarter, but I still have some annoying clues to solve for the part which is causing problems. However, another session might well lead to illumination.
My advice would be: concentrate on solving clues, enter the answers for which there seems no difficulty, keep a note of the redundant and missing letters, and be preparing to do a lot of erasing.