Non dom MPs - how difficult is it to change the law?

Top Tory donor and party chairman Lord Ashcroft has revealed he is still a "non-dom" but says he will at last give up that status. Leaving aside all the political mud-slinging about the timing of Lord Ashcroft's announcement I was intrigued by something I heard Justice Secretary Jack Straw say on the subject.

He was being interviewed on Channel 4 news about the change in the law required to ensure that all donors to political parties are domiciled in the UK. Mr Straw noted that the law had been changed a few years ago to ensure that all donors were registered UK voters. A further change is now required to ensure that they are domiciled here (or as Jon Snow repeatedly put it, rather simplistically, "paying full UK taxes").

Mr Straw blamed the delay on changing the law on HMRC as "the law [of domicile] is very complicated."

I'm sorry? The complexities of the law of domicile are an issue as between individual taxpayers and HMRC. Such complexities are irrelevant when framing an amendment to the law to deny political donations from non-doms. Indeed such a change would put the onus on donors to clarify their domicile status.

I note that a similar issue arises as regards a rushed amendment to the Constitutional Reform and Governance Bill. This will deem MPs and peers 'ordinarily resident and domiciled in the UK' for income tax, capital gains and inheritance tax. However it is reported that Peers would have three months from the change coming into force to ensure their status was in line with the new rules. If true this can only mean that they have 3 months to confirm their tax status from the beginning of the relevant tax year - if they were not ordinarily resident or non-doms in the previous tax year.