Just bear in mind through all of this that the Treasury hasn't named Barclays as the perpetrator. Barclays has been outed and sort of put its hand up, while casting doubt on the amount it has underpaid.
Also note that this is "aggressive", "abusive", tax avoidance, not illegal tax evasion.
The Treasury has clamped down on two practices.
One when a bank issues bonds or IOUs to raise money, then manages to buy them back at a knock-down price - but avoids paying tax on the gain.
The retrospective legislation the government is putting in place to fix the problem applies to this dodge.
The second appears to involve claiming tax rebates on investment income where tax wasn't paid in the first place.
This is being dealt with on a "from now on" basis by tightening up the rules on the tax treatment of the Authorised Investment Funds which were used.
Here's how HMRC describes the schemes...
"Debt buyback scheme
It is a normal commercial action for a company to buy back its issued debt that is trading at a discount in the market in order to reduce its liabilities.
However, the normal commercial profit that arises in such a case should be subject to tax and the corporation tax rules on deemed releases are intended to ensure that this profit is only exempt from tax in very particular circumstances where a business is in distress.
This scheme involves highly contrived arrangements to circumvent the intention of the legislation."
And the tax rebate scheme:
"Authorised Investment Funds
AIFs are pooled investment funds which invest in shares, securities and other asset classes and can have a range of investors from retail customers to institutional investors or financial traders.
The scheme aims to convert non-taxable income into an amount carrying a tax credit, thus generating a set-off or ‘repayment’ of tax to the scheme users that has never been paid. This is aggressive avoidance, since no tax is paid but a corporate investor could claim a substantial repayment."