IHTM36341 - Letters of offer: excluded duties
If there are any duties covered by the offer and other unpaid duties which are not included in the settlement your letter should contain a specific clause that lists the excluded duties. This should help avoid any dispute at a later stage.
If your enquiries on the case are complete and you are merely awaiting final figures in due course for estimated items, the clause should state that any additional liability arising on these items is not included.