Those who know me as the late Ken Macintosh will not be surprised to hear I was among the many scrabbling to file their tax return ahead of last week's deadline.

As I sent it off to HMRC I did wonder what difference it would make if income tax was entirely a Scottish affair, as is being proposed by some of those looking at further devolution.

I am very concerned about such a prospect. I believe that power should be exercised as closely as possible to the people affected. That's why I've always been a devolutionist. For areas such as defence, foreign policy and much of the economy, the most appropriate level is the UK. Devolution is not about seeking more powers, but about how we use those powers to transform Scotland.

The Scottish Parliament has had powers to increase or decrease income tax since 1999, but I am unaware of any proposals to use these powers. The prevailing wisdom seems to be that we should find a better balance between the public funds we spend and the revenue we raise. I find that a theoretical argument rather than a solution to a practical problem.

There is little evidence to suggest the Scottish Parliament has behaved irresponsibly on spending decisions. In fact, it is a legal requirement to present a balanced budget. Moreover, it is intriguing that those who argue Holyrood's accountability would increase with more powers over revenue fail to apply the same logic to local authorities, systematically stripped of any such discretion.

What worries me about this proposal is that it would radically reduce our tax base and put us in grave danger of devolving so many powers that we effectively became independent by default.

Tax revenue per head in Scotland, leaving oil to one side, is broadly similar to the UK average. But less comes from the taxes on income, wealth or property that those on higher incomes pay a higher proportion of, and more from taxes on spending including on drink, cigarettes and gambling. With more high earners in the south-east of the UK, Scotland benefits from the redistributive effect of a national income tax. Even assuming we negotiated a good deal that captured that benefit in a devolved income tax settlement, over time we would lose out from having shrunk our tax base.

In recent years, the share of all income tax paid by the highest earners has risen from around 20% to closer to 30%. With 18% of all UK higher-rate tax payers living in and around London compared to just 8% in Scotland, we would be cutting off our nose to spite our face by fully devolving income tax. It would not be fair to a devolved Scotland. It would be less redistributive and less progressive too.

I see the constitutional arrangements existing in equilibrium, a balance of shared local and national decision-making accepted both by Scots and by fellow citizens in the rest of the UK. The welfare state, our NHS, provision for pensions and much more depend on a sense of shared national interest.

Going too far in devolving tax or benefits risks fragmenting the system and breaking down the shared sense of our common future. For reasons of economic as well as political and social cohesion, we should continue to exercise joint decision-making on income tax with our fellow UK citizens.

More fundamentally, I do not believe we respond to the demands of those who support independence by constantly seeking further powers to devolve, but by pointing to the way we can transform Scotland within the existing settlement.

Just as we can transform childcare through the Scottish Parliament, a UK Government is better placed to break up our banks and take on the big-six power companies. Devolution allows us to resist a market-forces approach to health care while maintaining the integrity and strength of a truly national health service. The day after the referendum vote, we need to get on with the job of making Scotland a fairer, brighter and more socially just nation. We are full UK taxpayers and we enjoy the clear strengths of being part of the UK. Next year, I might even do my tax return early.