Last week the Conservative Party chairman, Eric Pickles, suggested that his deputy, Lord Ashcroft, would be "very happy" to give a media interview to clear up the uncertainty over his tax status. But that suggestion has been met with a flat refusal from the Conservative peer.

And silence, it would appear, is contagious. We report today on the refusal of the shadow Foreign Secretary, William Hague, to confirm what took place on a trip in 2007, partly funded by one of Lord Ashcroft's companies, to the Turks and Caicos Islands, where the wealthy peer has business interests.

There is no evidence of wrongdoing by Lord Ashcroft, but the lack of transparency that continues to surround his tax status is nonetheless worrying. He was awarded a peerage on the specific condition that he would become a UK resident by the end of 2000. Yet, almost 10 years later, the Conservative leader, David Cameron, will only say that Lord Ashcroft is "meeting" that undertaking.

Last week Mr Cameron said that it was "absolutely right" that the Tory candidate for Richmond Park, Zac Goldsmith, should rescind his non-domicile tax status and become a UK resident. So why allow this haze of ambiguity to linger over Lord Ashcroft's tax status? After all, Lord Ashcroft is a considerably more influential figure than Mr Goldsmith, channelling large sums directly into Conservative constituency offices in marginal seats. As for William Hague, his statement in the House of Commons Register of Members' Financial Interests makes it clear that he made the Turks and Caicos trip "in my capacity as shadow Foreign Secretary". There is a clear public interest in the activities of an individual who could be directing British foreign policy in a matter of months.

The row over MPs' expenses has demonstrated that secrecy and money are an unhealthy – and unpopular – mix in politics. If he is sensible Mr Cameron will make a virtue of transparency in the run-up to the next election, starting with the affairs of his shadow Foreign Secretary and his deputy party chairman.