Every day, Britain's schools need some 13,000 supply teachers - and paying for them costs the nation's schools a total of Pounds 300 million a year.
Ten years ago local authorities monopolised the market. Now, private supply teaching agencies are turning over some Pounds 90m a year, and that sum is predicted by industry experts to rise to Pounds 150m within a couple of years.
Capstan Ltd was established in 1992 by recruitment professionals Ray Mercer and John Heffernan. In its first year it had a turnover of Pounds 200,000. Today it sends 1,000 supply teachers a day to work in schools, has 15 offices across the country, and boasts a turnover of Pounds 15m.
"That's more than exponential growth - and in the next two years we're expecting to grow to a Pounds 25m business," says Mr Mercer, aged 31.
Bullish though he is, Mr Mercer does have concerns about local education authorities which do all they can to persuade schools to stay away from the private sector. Yet, under free enterprise-friendly New Labour, he feels happier than ever to vocalise his complaints.
"Bizarrely, under the last government we would have kept our heads down, " he says. "Now we feel confident enough to say we're not going to stand for it. And I just don't believe headteachers will allow things to go back to the way they were.
"Before the first agencies were set up in 1989, it was absurd that heads were having to go down a list of phone numbers at 7.30 on a Monday morning to try to get cover, because otherwise they'd have to teach the classes themselves.
"As headteachers become more commercially minded, the first thing they realise is that their time isn't free. It then makes sense for them to make just one phone call to us, and we do the rest."