Hi S_Pugh,
This is a portion of comment 84 :
Now you need a cipher. In fact you already have one, which you've created by writing letters against numbers when you solved Grid A. See if the remaining 34 numbers from Grid A make any sensible words using the possibilities from this key. No ? I thought not...but now say to yourself, what similar key could I use ? Don't over-complicate it, just think of the most basic key along similar lines to the one which just failed (26 letters, 26 numbers).
The problem that I had was to stop thinking in terms of one-to-many relationships (as in telephone dial) and get back to one-to-one relationships. How you then deal with the ambiguities is your choice...