ONE story has refused to go away this week and it's perhaps not the one you might have expected.

Despite apparently bigger fish to fry (Danny Alexander's warnings on an independent Scotland's membership of the EU, for example), the No campaign has kept chipping away at the SNP's plan to continue charging UK students for tuition at Scottish universities in the event of independence. Academics Together, an off-shoot of Better Together, set the ball rolling last weekend with a warning the proposal would "run into significant problems with EU law," which prohibits discrimination against fellow member states. The group's call for Alex Salmond to publish any specific legal advice he has to justify the policy was repeated by Alistair Darling in a speech on Thursday and by Labour's Ken Macintosh at First Minister's Questions. Various former Eurocrats and EU legal experts have emerged to cast doubt on the -plan and warn it would inevitably face a legal challenge, while Whitehall's latest Scotland Analysis paper yesterday said the plan was "clearly contrary to EU law".

The No campaign is pursuing this not because the fee-paying fate of English students in an independent Scotland is the burning issue of the campaign. Plainly it isn't (though the loss of UK fees would blow a £150m hole in the higher education budget). Rather, pro-UK campaigners see it as a question of trust. The First Minister was seriously embarrassed in 2012 after he gave the impression he had received advice from government law officers on an independent Scotland's membership of the UK, a suggestion that turned out to be untrue.

Mr Darling, who says Mr Salmond "has form" when it comes to legal advice, is hopeful of skewering him again and painting the First Minister as an unstrustworthy character. For his part, Mr Salmond is sticking to the stock response that ministers do not routinely divulge the existence or otherwise of formal legal advice. I've no idea whether Mr Salmond has such an opinion in his desk drawer but, having watched the story unfold this week, I'm pretty sure the issue has been discussed at some level with government lawyers. Nothing else could explain the peculiarities of Scottish Government's position.

Faced with demands at Holyrood to publish his own legal advice, the First Minister cited someone else's, Universities Scotland's in fact, as backing up his policy. Last March, the universities' umbrella body published advice from lawyers Anderson Strathern suggesting there "may" be a case, if the country became independent, for charging all EU students. "As a matter of EU law," the advice states, "it would appear that it may be possible to rely upon a residency requirement for access to preferential fees and grants regimes as long as that requirement is applied to all students regardless of their nationality and can be objectively justified." Objective justification, by the way, means the Government would have to satisfy the European court the policy was "necessary" and "appropriate" to achieve a "legitimate" aim. At the time, Universities Scotland director Alastair Sim said that, with a bit of work, the advice might form the basis of a policy that would get round the potential loss of income from UK tuition fees.

The Scottish Government, however, has not gone down that route. It says the principle of objective justification would provide an arguable case for charging UK students, hence the First Minister's claims, but ministers have rejected Anderson Strathern's view that all EU students, not just the English, Welsh and Northern Irish would have to be made to pay. On the face of it that's odd, given that Education Secretary Michael Russell has been trying for years to find a way to charge EU students who, because discrimination between states (though not within states) is banned, are entitled to free tuition just like Scots. Mr Russell's efforts to find a way round the rules have hit a brick wall so far but have not been abandoned, officials said, even though they will be if Scotland becomes independent. Perhaps the Scottish Government rejection of the (highly convenient) Anderson Strathern legal advice suggests its own lawyers have refused to endorse it. Exactly what they have said, though, remains a matter of conjecture.