They were told about the risks, but the temptation to gamble is so hard to resist, especially when you think you know more than you really do, and the price looks as though others have got it wrong. Yes, it’s the tale of the mug punters, who have seen the value of their bets tumble by a third or more, as shares in IG Group, CMC Markets and Plus500 all collapsed.

The surprise, then, is not the nasty loss but the valuation punters had put on these spread betting groups. Last February the Financial Conduct Authority publicly warned them about their behaviour, and now it has told them to break their nasty habits. Given that over four-fifths of the bets they take lose their customers money, in some cases much, much more than they imagined was possible, even the most supine authority might one day decide that we needed protection from ourselves.

The industry’s argument that foreign-based competitors would treat the customers worse looked unconvincing when last month Cyprus beat Britain to the regulatory punch on contracts for difference.

The FCA’s argument that so-called binary bets meet no obvious investment need might apply to all sorts of weird derivatives which seem to exist purely to further the interests of the promoters. Fortunately for the watchdog, binaries are classed as actual gambling, and are thus someone else’s problem.

Considering our national addiction to betting, we need a rather sharper regulator than the Gambling Commission to bring order to this slippery industry. Co-incidentally, a cross-party parliamentary group last week recommended cutting the maximum stake on fixed-odds betting terminals from £100 to £2. The group pointed out that the bookies have made £1.75bn this way in the last year, although it did not add that compulsive gamblers will always find another way to lose their money.

After the announcement CMC responded, through gritted teeth, that it “looks forward to working closely with the FCA over the coming months.” The mug punters like Fidelity, Legal & General and Henderson who bought the shares at twice today’s price can only hope they don’t lose much, much more than they imagined was possible.

Elementary, my dear Watson

The single market in goods and services with the rest of the European Union is jolly important, and we must fight to preserve it after we leave. Well of course; Britain is a trading nation, and the shuttling of goods across the Channel makes both sides richer. But services? London leaves the rest of Europe standing in financial services, but our continental partners, led by Germany and France, have successfully protected their domestic champions against British-based invaders.

The awkward truth is that there is no single market in financial services any more than there is in plumbing or child-minding. Each member state has its own rules, yet the myth has somehow grown up that this is a valuable single market in one of the few things we do really well, and is at risk from Brexit.

This myth has moved Stani Yassukovich, distinguished banker, former chairman of the Cirencester Park Polo Club and one of our finest imports, to suggest inserting an exchange in the forthcoming Sherlock series: “But Holmes, what was the significance of the single market in services in the EU?” “The significance, my dear Watson, is that there isn’t one.”

I promise not to pay the bearer

Those 1000 rupee notes you’ve been hoarding will shortly become worthless, when even the Indian central bank will refuse to accept them. Urjit Patel, the governor, explained that they remain as a liability on the balance sheet “as of now”, although if it refuses to honour them, it’s an odd sort of liability.

Perhaps Mr Patel has been studying the actions of the Belgian central bank. Its currency has long since disappeared into the euro. Holders of the lower denomination franc notes are told that the bank can’t be bothered with trivial sums, and declines to change them. Inflation may have nibbled away the value of our British pound, but at least the Bank of England does promise to pay the bearer on demand, however old the notes.

This is my FT column from Saturday