Since last Thursday the currency issues in the independence debate have been, perhaps deliberately, obscured by emotional language. As I've said, we really do need to distinguish between a formal currency union and the informal use of the UK pound by an independent Scotland.
A currency union's price is financial interdependence because each member state may create new money. In the Eurozone we see a practical example. The short version of the lesson is: when some members overspend, those which do not overspend have no effective choice but to bail out their partners or risk the collapse of the whole system.
The Eurozone is now being compelled by economics to pursue exactly the sort of political integration that is the opposite of the objectives of the 'Yes' campaign.
The failure of the UK to agree a currency union with Scotland would not lead to exchange costs for UK businesses. Such costs would in fact result from a Scottish decision to adopt a currency other than sterling, as I discussed on 13 February.
Informal use of the UK pound by Scotland would be unlikely to lead to much greater financial flexibility however, since we could only obtain more money supply by means of a balance of payments surplus / net inward investment.
Quoting my article of 7 November 2013, "My judgement would be that a Scottish currency is the least of the evils, but that it requires preparation to start yesterday and much statesmanship from Scottish ministers."