A SQUARE Sixties building that has recently been converted into student flats in the heart of Middlesbrough doesn’t look like very much. In fact, its ordinariness, with a pharmacy fronting onto Linthorpe Road, makes it appear utterly unremarkable.

But around the back of Scandinavian House – there’s a clue in the name – is an extension in which a community art studio is being established, and it contains a hidden historic treasure which has filled an Antiques Roadshow expert with such enthusiasm that he is coming to give a talk about it on October 24.

“It is completely fab,” says Andy McConnell, the Roadshow’s glass expert. “It is a very special thing – the only one I know of in this country.”

Again, from the outside, it doesn’t look too much: a drab, shabby wall made of more Sixties’ squares. But inside it explodes into vibrant colour. It is a stained glass window and the building is a former Sjomanskyrkan – a Swedish Church attached to a Scandinavian Seaman’s Mission.

“When people come into the building, they gasp in astonishment,” says Sara Calgie who has turned the disused church into an art studio. “It is really unusual.”

So unusual that some people are talking about the Sjomanskyrkan as Middlesbrough’s most important post-war piece of architecture.

Because the east coast faces towards Scandinavia, for a millennia there have been close connections between the tribes living on either side of the North Sea – perhaps too close for poor old St Cuthbert whose remains were chased around the North-East for two centuries by pesky Vikings.

In the great age of sailing and steamships, the first Swedish Church and seaman’s mission in the world was established in Hartlepool in 1885. It was a place for Scandinavian sailors (Norwegians and Danes also dropped in) to draw breath when they came into port.

Middlesbrough – a town founded on migration – had a mission from the late 19th Century, and once there were 12 similar institutions across the country, all in ports like Liverpool and the east end of London.

The Middlesbrough mission was expanded in 1963 when the Sjomanskyrkan was added. To make it feel truly home from home, the walls were plastered with a crushed rock carlite imported from Sweden and the main feature was a 10 metre wide modernist window designed by Christe Sjogren and made by the renowned Swedish art glass company Lindshammar.

“In the post-war decades, the Swedes were the richest people in the world and the grooviest – Simon Templar, the Saint, drove a Volvo,” says Andy McConnell. “Glass was one of the products of this extreme wealth. I think Sweden is the most important glass making nation of the 20th Century, and they held glass so close to their hearts that it became part of their national identity – more so than any other nation.

“With this building in Middlesbrough, someone wanted to make a cultural statement about national identity, just as we would hang a Union flag outside one of our buildings. I think it is amazing.”

Perhaps the most amazing part is that, made of pre-cast concrete, the window was imported as a single piece.

The Sjomanskyrkan closed in 2005, and the church was empty while the other parts of the mission were converted into new residential and retail uses. Sara, though, has brought it back to life with her Start Studio, which is run as a community interest company offering sessions in clay (Tuesdays), crafts (Wednesdays) and painting (Thursdays), and being an open studio the rest of the week.

Andy McConnell, who runs Britain’s largest antique glass shop in East Susses, is breaking his journey to record an Antiques Roadshow in Newcastle by coming to the studio to infect Teesside with his enthusiasm on October 24. His talk is fundraiser for the studio and the window, and it is entitled Swedish Glass: From Orrefors to Ikea. It starts at 7pm, tickets are £12 (including a glass of wine) and are available through eventbrite.co.uk.

The Start Studio is in Park Road South off Linthorpe Road, and the postcode is TS5 6HP. For further details, see its Facebook page.