Update Current Row based on Previous Row

I have a small table that need to keep a balance of work outstanding. The
fields are as follows

REC_COUNT (auto-index created record counter)
DATE (work date)
QTY_REQD (work added on that date)
BAL (balance of work outstanding at the end of that date)

The field REC_COUNT is used as a sort order field so that there is no
confusion about which record is next.

I need to be able to read the BAL from the previous record and add to it the
the QTY_REQD. I have tried using a query (I am not a VB programmer) using
'Dlookup' where the REC_COUNT is the current REC_COUNT-1 but cannot get this
to work. I have tested the theory of the DLookup OK by returning the
REC_COUNT from the previous record. I assume this works as the REC_COUNT
already exists for all records. However, the BAL field is null until this
query runs.

Can anyone throw any light on how I can achieve this with a query?

Regards

Tom