rrrobbo - I know what you mean. The hard part to this puzzle is not the arithmetic (which is straightforward) but remembering exactly what each clue is telling you (which I had remind myself time and time again. So, for any one clue.......
1. the clue number tells you where to enter the answer.
2. the clue itself (ie. the arithmetic expression) tells you what the sum of the distinct letters of the answer to this clue would be if the answer were written out alphabetically and any duplicates removed.
3. the number in brackets tells you how many letters in total there are if the answer to this clue were written out alphabetically.
So, in your example, it's M that consists of 3 digits (cells in the grid). G is the sum of the distinct letters of M, if M were written out alphabetically (ie. a string of 18 letters, given by the number in brackets) and any duplicates removed.
In the b clue, it's b that has 2 digits, not A+G (which is the sum of the distinct letters of b, if b were written out alphabetically (ie. a string of 9 letters, given by the number in brackets) and any duplicates removed
I found this all really confusing but, as long you remind yourself of 'the rules' for each clue that you look at, it does work out.