How can they use another country's currency?

Above:  statue of Ariel (from The Tempest) on top of a cupola at the Bank of England.

Earlier today I watched Andrew Neil's Sunday Politics which discussed what currency an independent Scotland might use.  The SNP insists they will use the Pound Sterling ("Sterling is a convertable currency and we would be using it" said an MSP interviewed on the programme - I didn't catch his name).  But how can they use another country's currency?  What are the mechanics whereby notes and coins are going to find their way into Scottish banks and cash points?  Are they going to buy pounds from the Bank of England?  If so, presumably they will have to pay for them in gold?

Or will they have their own version of the Pound (the "Poond" as suggested in yesterday's Guardian), print their own banknotes and fix the value to the Pound Sterling?  This will presumably cause them a lot of problems as Alex Salmond's administration is a big spender.  A PWC report in 2009 said that the public sector in Scotland was 50% of GDP - what will the credit rating agencies make of all this?

The SNP's strategy is to steer discussion away from issues such as the currency and focus on the process of the referendum.  For instance there is currently a lot of sidestepping about when they are going to talk to the Westminster government about the legality and fairness of the way the referendum will be run.  If there are to be extra questions on the referendum paper then surely a question on the currency must be a priority?