I was using the Sainsbury's cash point near to Highbury Corner Magistrates Court when the person in the queue behind me, who was clearly a lawyer, threw a huge tantrum.

He was aggrieved that I was using two debit cards instead of one. He explained that I was "selfish" and had "poor cash machine etiquette". He went on to explain that the correct approach would have been for me to have used the first card, then joined the back of the queue to use the second one, therefore allowing the queue to move at a steady pace.

In response, I explained that by his logic I should go into Sainsbury's, purchase my sandwich, join the queue again to purchase my Ribena and repeat the process for my apple. He told me that my comparison was incongruous, as people in the queue in Sainsbury's can see the items I intend to purchase and then make an informed decision over whether to join the queue I am in or opt for a different queue, making a rough calculation as to the likely waiting time based on the number of items in the trolley. He added that there is a legitimate expectation that people will purchase more than one item of food at a supermarket checkout, yet there is also an expectation that only one credit or debit card will be used at a cash point.

He did concede, however, that a person who had a full trolley during rush hour was equally as inconsiderate, particularly when online shopping was so readily available. To support his argument he cited the example of the "maximum 10 items" aisle.

I pointed out that this aisle was exclusively for baskets and not trolleys, making this reference point irrelevant. He explained that Sainsbury's would extend this to cover maximum items for trolleys in principle, but for logistical and enforceability issues. He would also need clarification as to whether two of the exact same product would constitute one or two items.

He said I had ‘poor cash machine etiquette’

I then pointed out the central flaw in his argument. His issue with my conduct was principally with the length of time it took at the cash point, which was not necessarily a reflection on the number of cards used. To highlight my point I referenced the elderly lady who used the cash point before me. She used only one card but as she printed a mini statement and took a while to remember her pin number, she took twice as long as I did.

I emphasised that the real issue was therefore the purpose the cash machine was being used for and, critically, the length of time spent at the cash point rather than the number of cards used or transactions made. Accordingly he had misinterpreted the definition of cash machine etiquette, if such a concept existed.

We both agreed that a sensible solution could be as follows: (a) Consistency and transparency on a national level on the interpretation of cash machine etiquette, with a select committee to look into the issue, primary legislation and the use of advertising campaigns and so on; (b) Enforceability at local level, for example the use of signs at individual cash machines, pamphleteering, an increase in the number of cash machine where applicable; (c) A reference to the European Court of Justice over whether there is a European Union directive already in place dealing with this issue.

While this discussion took place the person at the back of the queue complained that he had been waiting 24 minutes to use the cash machine. The irony was not lost on either of us.

Gary Conway is a criminal solicitor