I have thoroughly enjoyed this - it is challenging but seems fair enough. I am, however, stuck with the bottom right corner: 19A, 20D, 22A and 22D.
If I have the rest of it correctly, 3 of these 4 clues require the addition of a letter (i.e. they represent 3 of the 8 clues referred to in the instructions). I am pretty sure I know what those 3 letters are - but have no idea to which clue they belong. In particular 20A and 22D are driving me mad.
Any further clues gratefully received.