Rather a slog this week, definitely not a favourite, but got there in the end. Thanks to people here, not for any specific hints but more for a general sense that it could be done if I persevered.
I'm pretty unhappy with the way the "bcde" stage of the end-game was described. I identified the clues in question, then read and re-read that bit of preamble and it doesn't - in my view - express what is actually required. In fact, it expresses the precise reverse - I spent a goodly length of time in an Eric Morecambe-like state, having the right letters in the wrong order, hence putting the wrong numbers into the right calculation, whilst racking my brain as to what value of "a" could possibly produce a real word!