A park in Germany’s Freiburg. German euros could be worth considerably more than Spanish, Italian or Irish ones in the event the currency is abandoned

July 14: The woman at Barclays looks like she thinks I’m deranged. She has just sold me 100 euros in banknotes and I’m trying to explain about the serial numbers.

She glances nervously to check if there are some beefy colleagues nearby and decides I’m probably harmless.

I’m in a branch on Southampton Row in London, testing the theory that millions of British holidaymakers on their way to the Continent this summer are about to be fobbed off with potentially inferior euro bills.

Not forgeries, just bills that conceivably might turn out to be worth far less than others in the event of a full-scale abandonment of the currency.

Not all euros are identical. They each have a serial number — the first letters of which could determine the winners and losers in the event of a break-up.

A former Bank of England economist, Neil Record, a runner-up for the £250,000 Wolfson Prize, argues that if the euro is abandoned it will have to be done secretly and quickly and that the serial numbers should determine to which currency each note converts.

Euro notes are issued by the central banks of each member country. The serial number identifies which one.

While other economists contemplating the end of the euro believe that banknotes should be re-denominated according to the country they happen to be in at the time, Record — who runs the listed asset management firm Record Currency Management advising pension funds on $19 billion of currency exposure — says that the serial number approach is surer and better.

In essence, euros with an X-prefix, showing that they were German-issued, should become future marks. Euros with a U become francs. Greek-issued Y euros become drachmas.

Since the exchange rates will be allowed to float freely at this point, the theory is that the newly reinstated drachmas will plunge against the new marks.

So euros are not all equal. German euros could be worth considerably more than Spanish, Italian or Irish ones and twice or three times as much as Greek euros.

The question is: what kind are British holiday-makers ending up with? Could it be that our unloved banks are palming us off with the wrong kind of euros?

At Barclays, the cashier passes over seven 10s and six 5s. All the 10s are Xs. X is German. A good result. But what of the 5-euro bills, which are all Ps? Not Portugal surely, which last year was forced to seek a 78-billion euro bailout? No, P is the Netherlands, regarded as strong. If the guilder were to be reinstated, it would probably do rather well.

At a Travelex in Russell Square, all ten 10-euro bills I buy are X-prefixed. It’s the same at the Post Office in High Holborn. Opposite the British Museum at a Money Shop, most of the proffered bills are German; a couple are French.

The rates of exchange may be wildly different, with Travelex the worst value by a long way, followed by the Post Office, but the dominant euro seems to be the “German” one. That shouldn’t be surprising. Germany is issuing more banknotes than anyone else.

But it may calm any hot-headed Euro-sceptic who fears that Britain is being flooded with Ms (Portugal), Ss (Italy) or worst of all Ys (Greece).

Surely someone will sell me a Y? No chance at NatWest, whose Hatton Garden branch has no euros at all. Lloyds TSB sells me a couple of grubby-looking V notes. V is Spain. These could be worth a fraction of my Xs. The assistant at Lloyds in Cheapside in the heart of the City is the first to know about the serial numbers. “We had someone in the other day: he only wanted Xs,” she says.