My decision to help with the funding of Better Together arose out of a conversation with Alistair Darling on the Isle of Lewis last autumn.

I admired the fact that Alistair had agreed to give political leadership to the campaign to keep the UK together. We both knew that a campaign of this importance and scale would require significant funding. High profile donations to the SNP had been well publicised. I was delighted to be in a position to help.

My story is similar to many others who feel proudly Scottish, are very relaxed about our dual identity and do not wish either ourselves or our children to be forced into making a choice that I believe to be both unnecessary and undesirable.

Like many who worked with ICI, my father – who died only last year - moved south from his native Ayrshire because of his employment. He remained passionately Scottish until his dying day and brought up his family with the same values.

The nature of my work means that I am based in London, like tens of thousands of Scots now facing the same prospect of becoming foreigners in our own land. We have a family home in Scotland and my son has just finished university in Stirling.

When a great Scottish industry, Harris Tweed, was in crisis, I was proud to help turn things round by investing in a new company– considerably more, incidentally, than I put into Better Together! Hence my presence on Lewis and conversation with Alistair. Harris Tweed Hebrides has been a great success due to the skills and commitment of those who live and work there.

My usual business for the past 30 years has involved trading oil. This helps explain why, when it comes to the constitutional debate, my heart and head are in exactly the same place. I have found some claims made by the First Minister, frankly, astonishing.

Oil trading depends on managing risks around price volatility and production. Contrary to what some may think, success in my business is not achieved by speculating or taking chances. In fact, more often than not, the opposite is true. Wherever we can we try and bet on the sure thing.

We are a big company with lots of employees. It would be reckless of us to risk their livelihoods by gambling big stakes on what the future might hold. It is our duty to act responsibly. Sadly, when it comes to their language on oil, I see no sign of this responsibility coming from the Scottish Government.

A permanent decision about independence demands a serious longer-term view. Production of North Sea oil has declined sharply in recent years. There may be a blip on this downward curve from 2017 onwards with some new fields coming online, especially west of Shetlands, but no huge rebound is being forecast.

No oil trader or government can predict the price of a barrel of oil. Nor can we control it. Those of us around in the 1970s remember Middle East oil producers attempting to push up the price of oil by reducing production. With around 1% of world reserves in the North Sea, this is not an option open to us.

A crucial point - price per barrel and profit margins are not the same thing. In the same way, the price and the revenue for Governments vary vastly due to tax breaks and incentives involved. In short, talking about the retail value of oil – as the First Minister often does – is a red herring. It is the tax revenues that matter.

We have a spate of large-scale capital investments in the North Sea and potentially West of Shetland. However, these have only been made possible by targeted incentives from the UK Government. Field allowances make difficult extraction projects cost effective – but also impact upon revenues for the nation's coffers.

The oil futures exchange based on Brent Crude enables producers, governments and consumers to manage risks. A Scottish government could lock in, say, a 2020 oil price. But the price today on the curve for 2020 anticipates a fall from approximately $106 per barrel to $91 in 2020. The reasons are many: such as increased US production from shale oil and rising output from countries with huge reserves.

So Scotland could be leaving the UK against a background of declining North Sea production, declining prices worldwide, increased costs to extract, and increased costs to the exchequer to encourage investment. This doesn't sound like the oil boom Alex Salmond has been promising.

Oil represents 1.6% of UK tax-take but as much as 17% of an independent Scotland's finances. Yo-yo prices over the past decade have made oil and gas revenues the most volatile of the main UK tax streams. The impact on Scottish tax receipts, by removing the cushioning of a larger, more diverse economy, would be dramatic.

Companies seeking investment in oil have vested interests in talking up the long-term profitability but a small nation planning its finances based on oil should be more cautious. That is what the First Minister's own Fiscal Commission recommended - but seems to have been buried by the necessity to make a political case.

Norway, so often held up as an example, are planning on an oil price of $77 a barrel in 2014 compared to the SNP's 'cautious' estimate of $113. Make no mistake – such over-optimistic assumptions would come at a real cost.

The cash gap between peak and trough of oil revenues in the last decade is equivalent to the entire budget of the NHS in Scotland. I ask everyone reading this to consider one question. What if the Nationalists' optimistic projections are simply wrong?

I love Scotland and want only the best for the country and its people. I believe that Scotland gains most by having a distinct voice while able to share both opportunities and risks by being part of the United Kingdom. In short, I believe that we are Better Together.