It wasn't just the use of the first names. It was the smirk, that look of disdain that was most disquieting about Bob Diamond's appearance before MPs last week. The outgoing chief executive of Barclays had – for all the attempts of the broadcast media to hype the occasion in advance – nothing to fear. He knew he was not among equals.

I couldn't help laughing when TV correspondents predicted "tough questioning". Political journalists have been doing that for as long as I can remember and, unless my memory fails me, I have yet to see a single occasion when a parliamentary figure has made a single outsider squirm.

Over the past two decades prime ministers, on taking office, have announced that they want to give parliament more powers. The select committee system – so powerful in other countries – was the obvious route. Tony Blair promised to appear before the liaison committee twice a year, a group made up of chairs of all the various committees. And so he did. Each time he ran rings around them, with his jacket perched on the back of his chair and his schmaltzy smile. No matter how big the crisis of the moment, these supposedly experienced MPs never laid a finger on him.

On specific policy analysis they similarly struggle. Iraq? The Foreign Affairs Committee produced a lamentable performance in July 2003, letting the Government off the hook while berating the scientist Dr David Kelly, who later took his own life. Last summer's Culture, Media and Sport hearing on phone hacking – covered portentously by US networks – served only to give the impression that old Rupert wasn't so bad after all, particularly when he was hit by that shaving foam pie.

Then there are the inquiries that do not make prime-time coverage. Anoraks can follow them on the BBC's parliamentary channel. Sometimes, when the House of Lords is involved they can produce sensible, even original, findings. The joint committee on libel reform in 2011, chaired by Lord Mawhinney, was one such instance.

Other times, such as the joint committee on privacy hearings earlier this year, the lack of understanding of complex issues is embarrassing to behold. When I sat in front of the MPs and peers being questioned on the issue, I had to struggle to keep a straight face as the platitudes on internet policy reined forth. It came as no surprise that the final report managed in some important parts to be both belligerent and weak.

The culmination of these failures was the hapless questioning of Diamond and the fixing of the Libor inter-bank rate. With the exception of the Conservative former treasury adviser, David Ruffley, the banker was able to fob off his questioners by waffling and selective memory lapses. This tactic is now standard practice among witnesses. It is not hard to puncture it, but time and again they failed.

Given that the most important witnesses pay thousands to PR companies to prepare them for such events, it is surely not beyond the wit of MPs to get up on the issues and to role play in advance. What is required in these instances is sharp and informed interruptions, making it clear that nobody is taken in by selective forgetfulness. Instead, MPs seek to hide their failings through pomposity and grandstanding.

Some of the problems can be solved easily and immediately. The chairs of committees need to be far more assertive at the start of sessions, making it clear what tone of evidence they will accept and what they won't. The members should be warned to put party political allegiances to one side – easier said than done.

If the Coalition is serious about improving the credibility of Parliament, it should produce further reforms in the workings of committees. They should be given legal powers to subpoena, along the lines of the US Congress. Witnesses should be required, as has been the case at the Leveson Inquiry, to give evidence under oath.

The Leveson experience has been useful in demonstrating the relative merits and failings of both parliamentary and judicial inquiries. Even though the witnesses have been forced by law to attend and to swear to give the truth and the whole truth, some of them have either lied or been economical with the verite. The questioning by the presiding QC, Robert Jay, has been varied. At some points he has been forensic; at others, he and Leveson, seem to have been either star struck or credulous. It was frustrating to see Tony Blair and Alastair Campbell – two of the architects of the fractured relationship between politics and media – being given such a sycophantic free ride.

The important issue in getting to the bottom of the banking scandal is not who asks the questions but how? Lawyers are equally capable of being bamboozled, reinforcing a view that the establishment is keen to protect its own.

The failure of Parliament to get to grips with the many recent crises involving the misuse of money and power is exacerbating the original crimes and misdeeds of those involved. Politics has been seen, time and again, to be an ass. From spooks with their dodgy dossiers, to media moguls and their purchase of public policy through threats and inducements, to casino bankers, they have all got away with it.

The people supposedly representing voters, our members of Parliament, have publicly and contemptuously been swatted away. The nature of the power relationship has been laid bare for all to see. As a result, the already low credibility of our democracy has been further undermined.

After the expenses scandal, it is a stretch to imagine that politicians are capable of distinguishing between right and wrong on financial transactions. They are hardly equipped, as a group, to arbitrate on issues of probity. But in the end we have nobody else, unless we believe that all important issues must be sent to the courts.

Morality is one thing, expertise is quite another. The unpalatable truth is that those who have been running our big corporations and banks have got away with it, because they knew that they were always one step ahead of the politicians. My suspicion is that, once a few heads have rolled, they will return to their old ways. They know where the true power – and brainpower – lies.