Membership / account # only 8 digits
So I'm trying to set up an Avios shopping portal account. When I attempt to link to my Avios #, using the account # and my postal code (both 5-digit and 9-digit options), the website rejects it, saying my account # should be 16-digits (it's only 8) and should start with "308147". Further inquiry tells me that any email from avios will have my 16-digit account number. Nope. Only the 8-digit one that logs me into Avios.
Help?