[Stephen]

So if there is a yes vote in September. How will Scotland be able to keep the sterling pound?

Seen as all Scottish notes are not legal tender anywhere! All Scottish issued notes have to be backed up by English notes held by the issuing bank.

Scottish and Northern Ireland banknotes are not legal tender anywhere, The fact that these banknotes are not legal tender in the UK does not however mean that they are illegal under English law, and creditors and traders may accept them if they so choose. Traders may, on the other hand, choose not to accept banknotes as payment as contract law across the United Kingdom allows parties not to engage in a transaction at the point of payment if they choose not to.

In Scotland and Northern Ireland, no banknotes, not even ones issued in those countries, are legal tender. They have a similar legal standing to cheques or debit cards, in that their acceptability as a means of payment is essentially a matter for agreement between the parties involved, although Scots law requires any reasonable offer for settlement of a debt to be accepted

Most of the notes issued by the note-issuing banks in Scotland and Northern Ireland have to be backed by Bank of England notes held by the issuing bank. The combined size of these banknote issues is well over a billion pounds. To make it possible for the note-issuing banks to hold equivalent values in Bank of England notes, the Bank of England issues special notes with denominations of one million pounds ("Giants") and one hundred million pounds ("Titans") for internal use by the other banks.

So if allowed will these banks still have to hold English notes in their vaults, as the Scottish notes are really just worthless IOUs.