There are some questions politicians prefer not to talk about if they can possibly avoid it.

One is: can Scotland’s policy of free university tuition really be sustained?

Aversion to this question can be summed up in two words – Nick Clegg – though student finance was of course hugely sensitive in Scotland long before the UK Lib Dems spectacularly U-turned on their 2010 election pledge not to raise fees.

In the early days of devolution, the Labour-Lib Dem Scottish Executive introduced a graduate endowment scheme which kept tuition free but asked graduates to make a one-off £2000 contribution to fund bursaries for students from disadvantaged backgrounds.

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The ideas was vilified by the SNP in opposition, who (unfairly) badged it a “backdoor tuition fee” and made abolishing it a totemic pledge.

Ever since then, and especially following the rise in tuition fees in England and Wales, free university tuition has been a high-profile devolution dividend, held up as one of the great benefits of living in Scotland.

Which indeed it is – though not one without difficulties attached. That headline generosity and egalitarianism comes at a cost – a financial cost and, some fear, a cost to certain would-be students.

Some £2,020 less is spent per head each year on Scottish student tuition than the amount available for English students taught at English universities, according to a report by the Institute of Fiscal Studies last month (though Scotland’s four-year courses mean overall funding........

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