Dear Charles,
Still stuck? As I said before this is based on the idea of the ADFGVX cipher, but instead of starting with a 6x6 grid it starts with a 3x3x3 grid in which the 26 letters of the alphabet and the underscore, representing a space, are entered in some order. This encodes those twenty seven characters as triples AAA, AAB, AAC, ABA, ABB, ABC, ACA ACB, ... and so on.
Harry
PS. The rest of the clues will be published at 3.15 on the 27th, 28th, 29th and 30th Dec. all the best, Harry
