Your invoice table should NOT contain the work location.
Sorry, but it just shouldn't.
You're using a relational database - use it relationally! There should be a
Locations table with a LocationID and other (atomic!) fields: CustomerName,
AddressNo, Street, Postcode. Your Invoice table should store *only* the
LocationID; this can easily be inserted using a Combo Box on a form,
displaying enough information to identify the location but storing the numeric
ID.
If you're using table datasheets to enter data... DON'T. That's not their
function. Instead use a Form with appropriate tools - subforms, combo boxes.
listboxes, etc. And if you have Lookup Fields defined in your table, you
should be aware that most developers dislike them intensely: see
http://www.mvps.org/access/lookupfields.htm for a critique. You do want to use
the lookup *capability* for Customer and Contractor information, just as
suggested for Location information - but you can do so on a Form, without
needing the disadvantageous features of the Lookup Wizard.
John W. Vinson [MVP]