Not long ago it seemed noteworthy that a millionaire was no longer the owner of assets worth £1 million - or even $1 million - but someone who receives £1 million a year.

If the private equity boom goes on much longer, billionaires will be going the same way.

The founders of Blackstone are set to collect £1.3 billion between them when their New York private equity outfit floats on Wall Street shortly. They have been doing quite well already. Steve Schwarzman, Blackstone's chief executive, received $398.3 million dollars for his efforts last year, filings with the Securities and Exchange Commission reveal. His senior chairman Pete Peterson earned $212.9 million - not bad going at the age of 81.

Christopher Mills and the bunch of nimble investors who bought Dowding & Mills 18 months ago may not be in that league. But they have demonstrated that private equity works in Birmingham, too.

Peter Hollings, one-time chairman of D&M, is not alone in being sickened and perplexed at the prospect of shares in his former company coming back to AIM trading at close to 100p, just 18 months after they were taken private at 20p.

True, that is comparing apples and pears. The new shares in Castle Acquisitions, the "shell" that is making a reverse takeover of the re-named D&M do not replicate those bought out in December, 2005.

Castle is bringing £12 million of pension money to the party - and D&M is paying £29 million to buy in its preferences shares. Properties have been sold to raise £7 million along the way. There has been a lot of borrowing and now a lot of repaying.

This is not arithmetic intended to be understood by anyone other than the perpetrators.

What is abundantly clear is that the original shareholders were stranded on the sidelines, while Mr Mills and his friends have done spectacularly well. D&M has prospered under their stew-ardship - partly because the engineering industry's unforeseen revival unleashed a surge in demand for its services. What was a tightly stretched pension scheme is to be soundly funded.

Mr Mills points out that projects like this are risky. Some of them fail. They are also the by-product of cheap and available money.

Private equity fortunes do not drop out of the sky, nor entirely from tax breaks.

They are forged by seeing opportunities and borrowing somebody else's money to grasp them.

This is rarely a matter of old-style asset stripping. It can be a beneficent exercise, but by its nature a deeply secretive one that arouses suspicion.

Blackstone's filings reveal that private equity fortunes can be in a different league to other fortunes.

That kind of money raises questions like 'Where did it really come from? Who is poorer?'.

Plainly it is better that Dowding & Mills should be a success rather than a failure. If it was in financial straits it might never have been able to join in the industrial revival to any great purpose.

Now we shall see if it is coming back to the stock market at the top of a short-lived boom.