Thanks picastoc, but the Line 8 on the form sent by AST is not...

Thanks picastoc, but the Line 8 on the form sent by AST is not the Date of Birth. When you google W-8BEN, there have been different IRS versions of this form, depending upon the revision. AST are using an old 2006 revision. The Reference number they are asking for is the Account number (the AST Account number). Presumably this is one of the very small long numbers on the front of the form (either the one above our name, or on the barcode at the bottom - i.e starting with 900000). However, I'm hoping they will respond to confirm. The autoresponse email said they will be in touch within 48-72 hours (pretty slack).

If I don't hear back by email, I will call them.

Hopefully I will then be able to scan and email the form to someone there to confirm all looks okay before I send the original by snail mail.