In the airless committee rooms of the Houses of Parliament something remarkable is happening.

The directors of the failed HBOS are being made to account for their actions.

It is four years since the unlovely conglomerate formed by the merger of Bank of Scotland and Halifax collapsed – probably the biggest total failure in British history. By the time Lloyds, which unwisely took over the bust bank in 2009, has worked through the pain 40,000 people will have lost their jobs, £45 billion in loans will have been written off and two million small shareholders will have lost most of their investment.

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That doesn't count the damage to pension schemes and the billions pumped in by taxpayers.

Yet until now no-one has called all those responsible to account. The Financial Services Authority has sought to throw all the blame on one man, Peter Cummings, who led the HBOS corporate department. He, undoubtedly, was responsible for a large share of the duff loans, but as the hearings before the Parliamentary Commission on Banking Standards unfold, it is clear the "rogue trader" argument is not an adequate explanation for what caused the disaster.

Several witnesses have pointed to the need for Mr Cummings to get sign-off for big or unusual transactions from a second director or the chief executive, and heavy pressure was put on him to up his profits and lending at a time when the economy was in free-fall. Other losses had nothing to do with Mr Cummings.

The commission has touched on HBOS international operations, where huge debts were incurred in Ireland and Australia, and on the treasury department, which bought US "liar-loan" securities. It has yet to examine the retail operation where there were losses on unsecured personal loans and 125%, self-certified and buy-to-let mortgages made up a quarter of all home loans, totalling £60bn.

The commission's HBOS hearings are unusual. They have a barrister, David Quest, doing most of the questioning. He has done his homework and gets further than MPs on select committees do in their clamour for soundbites. But sadly this is not a court of law, or even a Leveson-style judicial inquiry. Mr Quest cannot cross-examine and sometimes has to let witnesses off the hook.

Former finance director Mike Ellis was allowed to say "I wasn't there, so I can't comment" far too many times – even claiming he could not talk about the £7bn of provisions made for 2008 because he was not employed when the accounts were "struck", even though he was there for the whole of the year in which the losses were incurred.

The chairman, Lord Turnbull, who in his former role as Cabinet Secretary coined the infamous phrase "economical with the truth," asks few questions, but when he does he gets right to the point.

More than once, faced with a self-serving explanation of all the committees, reports, checks and balances in place at HBOS, he has murmured aloud: "Makes you wonder how you got it all so hopelessly wrong."

Some witnesses condemn themselves. Sir Charles Dunstone, head of Carphone Warehouse and HBOS non-exec, described the bank as having a very transparent culture and said the quality of the information given to directors was very good. Yet the scale of the losses had taken him totally by surprise. "Hard to reconcile those two positions," remarked Mr Quest dryly.

So far I have heard nothing which changes the conclusion I came to in my book that it was the obsession with growth at any price which brought down the bank. The fact that it kept lending in 2008 when other banks had stopped was enough to cause the international money market to cut off the wholesale funds which were its lifeblood.

George Mitchell, who resigned as corporate banking director of HBOS in 2005 after failing to get the top job, put it succinctly: "Once you have spooked the market and confidence in your name has gone, it doesn't matter if you are trying to raise £50bn or £150bn, you are not going to get it. The market may be global, but it is a veritable village when it comes to gossip and rumour."

l HUBRIS: How HBOS wrecked the best bank in Britain, by Ray Perman, Birlinn, 2012.