Who estimates these things? Search me, but they further estimate that 17 of these flushes need only be short, four-and-a-half litre ones; the full nine-litre flush is only required three times. You should be able to choose between a short and a long flush if you have a cistern with a little sticker saying something like "short flush - pull and let go; full flush - pull and hold".
But a surprising number of these dual-flush cisterns are nothing of the sort, because the installing plumber failed to make one final adjustment --he didn't remove a little piece of rubber from the siphon.
If you think you have a dual-flush toilet, but can't tell the difference between holding on and letting go - if you get my drift - then have a look in the top of the cistern. You may have to undo one or two plastic screws to get the lid off. Find the siphon, which is the big plastic tube thingy in the middle, and have a look around the sides of it. There should be a little rubber bung stuck in a hole, below the water level. If you pull it out and have a look, it may even bear the legend "remove this bung for dual flush".
Now watch what happens when you pull the lever and let go - the water will fall to the level of the hole, air will be sucked in, and the flush will stop. If you keep the lever held down, the piston blocks off the hole and all the water is siphoned out. Brilliant. So why did the manufacturers bung up the hole in the first place?
The reason is that when the cisterns are installed at low level, the short flush is sometimes too feeble, even for those 17-times-a-day situations. So in toilets used by the great British public the building managers usually opt for a constant long flush. In fact this has been so common that, just when we were getting the hang of them, dual-flush cisterns are being phased out; all new cisterns are smaller - seven-and-a-half litres - and give a full flush every time. Since we are supposed to be trying to save water, this does seem like something of a step backwards.
Low-level cisterns are also responsible for another problem. Have you ever been in a toilet where the seat will not stay up? This can be very dangerous for men, especially drunken men in strange toilets. Stop that sniggering; you know what I mean.
This hazard usually results from the original high-level cistern being replaced with a low-level one, but without moving the pan forward. So where there used to be room for the lid and seat to be raised past the vertical and leaned against the flush pipe, now the cistern is in the way. If you have this problem at home then I suggest you get it fixed, at least before your next party; legal claims for that type of personal injury can work out very expensive.
Still, if you do have that toilet problem, at least you will have a greater chance of staying happily married. Apparently, men leaving toilet seats up is one of the most frequently cited complaints in divorce proceedings. Who estimated that? Search me, but all the women I've mentioned it to agree that it must be true.
q You can contact Jeff Howell at the Independent on Sunday or by e-mail on: Jeff@doctoronthehouse. demon.co.uk