You need to also file the Form 8821 at the same time as Form 706-NA so that the US IRS will correspond with you as the filer of the Form 706-NA.
Even though incorrect, the US IRS prefers to see a “Zero” on lines 12, 14 and 16; and on line 7 you insert the Unified Credit Amount. This will prevent you receiving a tax bill that has then to be cancelled under the UK/US Tax Treaty.
Restricted Transfer Certificates are also no longer available. They are now only issued when US Estate tax is payable.