Jamesah - as I mentioned above, start by almost completely (see my second paragraph) completing the third grid. Start this by concentrating on A,b and G, moving on to the two main possibilities for D and its associated entries. Once the third grid is filled, move on to the second. This can be completely filled without ambiguity before moving on to the first.
However, since completing this puzzle I have noticed that there are (at least 2) alternative entries possible for B and c in the third grid - please correct me somebody if I am wrong. I think c in this grid is (prime x square) and as such the square could be 64 or 4, making c start with a 3 or 8 respectively. I expected one of the possible answers for B to be excluded by its appearance in one of the other 2 grids, but this does not seem to be the case. I shall of course recheck all answers carefully and report back on this one .