They have a new little reportette out. Their instructions to George Osborne on what to do in the Budget. And it contains something of a howler. I've been chatting to them this morning about it and still haven't managed to get them to see what they've done wrong here.

The scale of Britain's growing inequality is revealed today by a report from a leading charity showing that the country's five richest families now own more wealth than the poorest 20% of the population.

This is true of course. But it's absolutely nothing at all to do with increasing inequality. It's also nothing at all to do with the calculations that they use:

In a report, a Tale of Two Britains, Oxfam said the poorest 20% in the UK had wealth totalling £28.1bn – an average of £2,230 each. The latest rich list from Forbes magazine showed that the five top UK entries – the family of the Duke of Westminster, David and Simon Reuben, the Hinduja brothers, the Cadogan family, and Sports Direct retail boss Mike Ashley – between them had property, savings and other assets worth £28.2bn.

They got their numbers on the wealth owned by the poor from an IFS report. Which was actually about income inequality, not wealth. And they've then managed to overlook something really quite important.

If you've no debt and a £10 note then you, yes just you on your lonesome, has more wealth that the bottom 20% of British society in its entirety.

For it's possible to have negative wealth, d'ye see? In fact, negative wealth is a normal part of every life cycle. That newly minted graduate carrying student loans? Highly likely to have negative wealth. You've just splashed out on a new car on HP? The moment you drive it off the forecourt the depreciation means the debt is greater than the value of the asset: if you've no home equity then this could be enough to make your wealth negative. You can have a very nice income, hundreds of thousands a year even, and carry debt higher than the value of your assets. Sadly it's not true that all of us will have an income of hundreds of thousands a year but except for a lucky few it's almost certain that we will at some point have negative wealth.

Given the lifecycle of wealth it's entirely normal, possibly even entirely desirable, that the bottom 20% of the population in wealth terms will have negative wealth.

So Oxfam is actually correct in their assertion here. But entirely wrong in the method by which they've reached it.