James
The attached db is a copy of the one you posted with some amendments, to show how you might get the effect you wanted on your form.
I have added the PK field [Customer Number] to the Record Source of the form.
I have made the form open at a New Record.
I have put code in the After Update events of the "Postcode" and the "DoorNo" controls. This code looks for the existence of a record with the same PC And DoorNo. If found, it filters the form to only show that record and removes the entered details from the new record.
I don't understand your table structure or the actual purpose of the db but I hope this helps in some way