Alan,

>I am converting an inherited system from an Access database to FB and am
>faced with this problem, because Access allows "incorrect" use of SQL and
>allows for 2 tables in the update statement. I would be grateful for any
>help because the principles in this apply to a number of other projects
>(and future ones I guess)

You can reference other tables in an update statement. E.g.

UPDATE POSTINGS
SET Total=0
WHERE exists (select 1 from LEDGER
where LEDGER.Balance=0
AND POSTINGS.Acno=LEDGER.Acno)

does work.

HTH,
Set