A few days ago, I took a phone call from a friend, a woman in prison. She is nearing the end of a lengthy sentence and she was very angry. She said that, during her time inside, she has met dozens, possibly hundreds, of women jailed for benefit fraud. Many cases involved relatively small amounts of money and were carried out by women in serious financial difficulties. Women who, for instance, had continued to claim benefit while doing cash-in-hand work as cleaners, child-minders and other black economy jobs. Again, in most cases, these women used their ill-gotten gains to feed and clothe their families, not to live the high life. Others have been guilty of "living together" offences (the second most popular benefit fraud) claiming single persons benefit while living with a partner who is working.

What was the difference, my friend asked, between these women and the MPs who exploited their expenses system? A good question and one which has been on my mind, almost since the first day the expenses earthquake erupted.

They say it takes one to know one. As a former thief, I also have a question to ask: why is it taking so long to instigate criminal proceedings against those allegedly involved in the worst cases?

Without naming names – and prejudicing possible future trials – if a person knowingly claims taxpayers' money against an expense that has expired, or been grossly exaggerated, that represents as plain a prima facie case of fraud as could be imagined. Yet the word fraud was not mentioned in the first few day of the fuss. Then, when the notion of criminality – as opposed to over-greedy snouts grubbing deeper into the trough – was mooted, we heard that the police were to meet with the Crown Prosecution Service to discuss the matter. Today, the police have ruled out a broad inquiry, but say investigations may continue only in a small number of the most extreme cases.

Back to my friend in the slammer. She says that most of the women benefit cheats she has met there were arrested and charged within days of their offences coming to light. No lengthy conferences between plods and prosecutors to decide whether there was a case to answer, they were nicked before they knew it.

No surprise either, that so many benefit criminals end up getting caught. Successive governments have spent millions cracking down on those fiddling the welfare system. A national network of hotlines encourages, with cash rewards, citizens to grass a neighbour they suspect of breaking benefit rules. (Was there a hotline in Westminster?) And last year's welfare reform bill introduced telephone lie detector tests to flush out even more fiddlers.

Most benefit fraudsters plead guilty, they knew they were breaking the law but, in the extreme hardship cases, saw no alternative method of providing for their children. The judges and magistrates who sentence such people invariably proclaim the guilty had "abused" the welfare system. Not all first-time offenders go to prison – though some do – but for repeat abusers, jail is inevitable. I wonder how many MPs can claim to be guilty of a singular offence?

My friend asked what the difference was between those banged up for benefit fraud and the right honourable ladies and gentlemen who have been fleecing the system so dishonourably? The answer surely, in most cases, is the difference between need and greed.