The owner of a 19th Century painting of Scotland’s bard Robert Burns – which is thought to conceal hidden Masonic symbols – has revealed he bought the portrait, valued at £2 million, at auction for under £1,000.

Stephen Barnes, 67, kept his identity secret since acquiring the work by artist Alexander Nasmyth and was prompted to speak out in an exclusive interview with the Sunday Herald after deciding it should be bought for the nation.

The painting was lent to Dumfries House for 18 months but is now under lock and key in a bank vault. Barnes, who is from Hull, has exchanged letters with former First Minister Alex Salmond about the portrait and now plans to write to his successor Nicola Sturgeon.

He said: “I think the painting should be in Scotland. It’s of historic interest and I think it should be seen. I know that Nicola Sturgeon is very much into Robert Burns and this painting would be ideal for Bute House. I haven’t pursued it as yet but I am going to write to Mrs Sturgeon. I don’t think she’ll have many bawbees in her purse but there may be some well-heeled Scottish SNP supporters who may want to buy it for Scotland. I don’t intend to flog it at Ingliston Market, put it that way.”

The Herald: The 19th century portrait of Burns by artist Alexander Nasmyth

Barnes said he “sold jumpers on a Saturday” at the market during a 15-year spell north of the border, when he also worked for Atlas Cranes. He clearly has an eye for a bargain and “took a punt” on the painting when he saw it at an English auction house, covered in thick dust.

He said: “I had a good feeling that this was an old painting. I took a gamble. It looked right. The picture itself, original canvas. The original frame. The auctioneers didn’t recognise it. There’s so much stuff that goes through. They give it a quick glance. They don’t go to academics like Dr David Mackie (a leading scholar of Scottish painting based at Cambridge University), who examined all of the Burns paintings and authenticated mine in 2013.

“I don’t want to say how much I paid for it but it was less than £1,000. The frame alone is worth what I paid.”

The painting is believed to be the fourth version of a portrait of Burns by Nasmyth painted in 1787. Other versions hang in the Scottish National Portrait Gallery in Edinburgh, the Kelvingrove Art Gallery in Glasgow and the National Portrait Gallery in London. Friday was the 221st anniversary of Burns death, on July 21, 1796

After Barnes cleaned up his painting – thought to be a copy, made by Nasmyth himself, in the early 19th Century – he discovered small letters and symbols invisible to the naked eye.

Burns expert and writer Jerry Brannigan believes the tiny marks were deliberately placed as Masonic symbols in the paintings by the artist. He presented his theory on the 'Da Vinci code of paintings' at a conference held by the University of Glasgow’s Centre for Robert Burns Studies at the Robert Burns Birthplace Museum in Ayr in January.

Barnes said: “I think there are lots of Masonic symbols. There are rows of numbers which I was told could well be lodge numbers or personal Masonic numbers, given that Burns and Naysmith were in the masons.

“A unique one-off of Robert Burns, it’s very desirable. And once you throw the Masonic stuff into the mix it becomes even more desirable. The only way you can describe it is as a diamond in the rough and Burns was a rough diamond. It’s very difficult to quantify the value. The antique trade gazette projected a figure of £2m, and that was some years ago. It could be worth more.”

Barnes, who describes himself as “a pauper”, added: “Maybe Nicola Sturgeon could go to some people with deep pockets and ask them to buy it for Scotland. I’ve done my bit for Scotland [by lending the portrait to Dumfries House] and I can’t do any more. If it ends up overseas then, well, so much for Scottish tourism.”

A Scottish Government spokesman described the portrait as an “iconic painting” which has “long been recognised the world over as the image of Scotland’s national Bard". However he pointed out that the Scottish Government “does not collect art works”.

The spokesman added: “The future of the painting will hopefully allow public access to continue.”