I want to run a query which finds all dates that are due in the next month/week

ie. Expiry Date: 17/12/01
Expiry Date: 19/12/01

I need an expression which does this for me?

You should be able to use the DateAdd Function in the criteria on your Expiry Date field.

<=DateAdd("d",7,Date())

will find all records that are due in the next 7 days.

HTH