Khalid Khawada, who runs Westcliffe News at Westcliffe-on-Sea, Essex, had a nasty shock during a recent expedition to the Southend branch of his cash and carry, when the cashier found six fake £20 notes. When she first checked, with a detector pen, they appeared genuine, but when she put them through the blue light detector machine they registered as fake. (He says she was an unfamiliar face so he assumed she was new and therefore being ultra careful.)

Khalid says: "They looked so good. They were embossed Bank of England. I'm 99% sure they were not fake."

He added that some of the notes had a little brown tint near the silver strip. "This is because we had to iron these notes before putting them in the ATM and so they had slight scorch marks."

He called the police to the branch. "They looked about 23- to 24-years-old whereas I have 30 years' experience with notes. Even the banks rely on the look and feel of notes. They don't use these machines, but all the police did was check the blue light device."

The upshot was that the branch confiscated the notes and said they would hand them over to the police. Khalid was not happy with this because he says that the Bank of England, receiving forgeries from all over the country, would merely destroy the notes without proper investigation. (That's not the case, according to  the BoE. "All counterfeits received undergo thorough investigation and, if found to be genuine, we will reimburse the individual concerned," a spokesman said.)

"Two machines, different results," says Khalid. "I spoke to head office and they said sometimes the pen doesn't work. Now when I go to the cash and carry I take only £10 notes."

It is true that dud £20s outnumber tenners. During calendar year 2008 the number of counterfeit Bank of England banknotes taken out of circulation was about 686,000. Of these only 5,000 were tenners whereas 675,000 were twenties, far and away the most popular of forged notes and no doubt because genuine £20 banknotes have the highest circulation. The Bank of England says that, compared with the average number of genuine banknotes in circulation of more than 2.3 billion, the incidence of counterfeiting remains very low. The face value of the counterfeits removed from circulation in 2008 was £13.7m.

All very interesting, but it doesn't help Khalid. What's your experience on this? After years of handling notes you must feel, as he does, that you can spot a fake at 50 paces. Any tips you can hand on? And how many of you have had notes confiscated by your cash and carry? And how reliable do you find detector pens to be?