Robomatic; What I did not state in my earlier post is that although Scottish banknotes are not legal tender, they are generally accepted as they were always issued on the understanding that the issuing bank held valid Bank of England banknotes in their vaults sufficient to cover all the notes they issued. (these days it may merely be a requirement to hold a balance in an account with The Bank of England for this sum) In this way, it is always the case that the holder of the Scottish banknote could always insist that the issuing bank exchange it for 'real' money.
In much the same way, the Bank of England (when it first started issuing banknotes) held a sufficient depository of gold to be able to repay these 'promissory notes'. It was only in this way that banknotes became acceptable, otherwise people would have insisted on having their money in gold. This situation ended when England went off the 'Gold Standard' in 1919