FOR SOME 30 years now the British have been victim to a nice little earner. They have been told their homes suffer from a mysterious ailment called rising damp, which will cause untold damage unless the walls are injected with chemicals and the plaster is hacked off and replaced. Since this problem usually comes to light when a property is being sold and bought, and since its rectification will be a condition of the mortgage, the pressure on the buyers to go along with the rising damp prognosis is immense.

So they pay up - anything between pounds 600 and pounds 6,000, depending upon the direction of the wind - and the new home is hacked about, drilled, injected and replastered. And so it goes on, day after day and house after house - the damp-proofing fraud spreads to every street across the country.

But now at last a BBC TV programme, to be screened tomorrow night, has grasped the rising damp nettle, and looks set to make waves that could end up wetting the shoes and socks of the whole property industry.

Some people in the construction industry have always complained that the damp-proofing game was a con, but their voices were drowned out by the marketing hype of the damp-proofers, and overruled by the mortgage lenders, insurers, estate agents and surveyors, who looked on and fiddled while the plot developed. These sharp-suited professionals, and their associations, now find themselves up to their elbows in it. You think the mis-selling of private pensions was a scandal? The mis-selling of chemical damp-proofing probably outstrips it by a factor of 20; at least five million British homes have been subjected to this wasteful charade. So who is to blame?

The damp-proofing train is usually set rolling by the surveyor who inspects a property when it is being bought. The Royal Institution of Chartered Surveyors always tells the public that the normal mortgage valuation survey should not be relied upon as a guide to the condition of the place, and recommends the more detailed "homebuyer survey and valuation". Unfortunately this type of survey requires the surveyor to check the walls for evidence of "dampness", and this is where the trouble starts.

For what happens is that the surveyor prods the walls with a small battery- powered instrument which measures, not moisture, but the ability to conduct electricity. And since wallpaper and plaster are often electrically conductive, this means that the meter can register a reading, even when the wall is dry.

What follows next will be familiar to practically anyone who has ever bought a home: the surveyor recommends further investigation of the suspected "dampness" by a firm which makes its money by selling, er, chemical damp- proofing.

The damp-proofers' trade association has long insisted that its members are highly trained experts, who use electrical moisture meters as but one tool in a catalogue of investigative techniques, and that they diagnose rising damp only after carefully plotting a pattern of meter readings over the whole surface of a wall. But tomorrow's programme exhibits a very different picture, as it shows a succession of ordinary blokes poking a wall once or twice with a meter, and solemnly pronouncing the presence of rampant rising damp.

If you have a video recorder, make a tape of this programme to show your friends and neighbours. And above all, keep the videotape in a safe place, to show to your surveyor next time you move house.

Jeff Howell appears on `Raising the Roof' tomorrow at 8pm on BBC2.