Summing a selection of lines in a pivot table

Hi all,
Probably a totally newbie question on pivot tables but I can't seem to
find out how it works. Here's my problem:

I've got an Excel worksheet that (coming in from jet reports) contains
something like

#Asset 1 Description A1 Total A1 Mapping A1 = X1
#Asset 2 Description A2 Total A2 Mapping A2 = X1
#Asset 3 Description A3 Total A3 Mapping A3 = X2
#Liability 1 Description L1 Total L1 Mapping L1 = X3
#Liability 2 Description L2 Total L2 Mapping L2 = X4
#Liability 3 Description L3 Total L3 Mapping L3 = X4

This last field, mapping, is what's important for my pivot table; it's
the asset/liability id but in a different system. The first column is
unique in the excel file, the last column can be duplicated many-to-
one.

Currently I've got a pivot table set up like:
X1 Sum X1
X2 Sum X2
where X1 is the mapping of both A1 and A2 and X2 is the mapping of A3
for instance.

This works fine but I need to add subtotals like so:
X1 Sum X1
X2 Sum X2
[subtotal: X1+X2]
X3 Sum X3
X4 Sum X4
[subtotal: X3+X4]
Complete total

In other words I need to sum a couple of lines in a pivot table
according to a fixed schedule (grouping assets and then grouping those
subtotals to a total of assets etc.)

Thanks in advance for any help,
Chris