I hope I can explain this.

I have a subsummary part which creates 3 groupings each with summary total. I want create a grand summary which sums the three - less one particular grouping.

eg

result; total-bbb (=8)

aaa...3

bbb...5

ccc...5

Try Fast Summaries, there is a blog post on it here: http://www.filemakerhacks.com/?p=2098

I think I have sorted it -

create a "key" value to match the summary to exclude,

make new TO based on the group and the key.

Use getsummary based on this TO.

create a "key" value to match the summary to exclude,

make new TO based on the group and the key.

Use getsummary based on this TO.

Not sure what "getsummary based on this TO" means. In any case, relationships ignore found sets - so using a relationship in a report based on a found set may not be such a good idea.