I finally finished this last night, having at first thought I’d finished the night before. My problem then was that my number count wouldn’t tally with H, even though I was certain that I knew *. So, I carried out doing the letter change and found it to be reasonably readable, just having the wrong letters for the start of words 2 and 3. This pointed me to the area where I had gone wrong, so I spent an hour or so doing various calculations until everything added up.
So, my advice to anyone thinking of giving up is to try the letter change starting from the bottom and working backwards. It worked for me.