subtotal

Hi
Can't work out how to do this so need some help I am afraid!!
On a list page I have the record sorted by activity_id. I would like to add a heading for each adtivity_id group and a subtotal of the activity_value field. Something like...
group 1

1 dave £10

1 pete £20

group 1 total £30

group 2

2 tom £30

2 Ian £40

group 2 total £70

grand total £100
I can group and total on the sql editor but then all the records don't show as well.
Hope you can help - thanks
Gareth