“As a prewar building it must have blown people’s minds,” says Jack Hale, admiring the glassy magnificence of the Express building in Manchester. “In the same period you were still getting stone, classical buildings and then this appears.”

His colleague Eddy Rhead agrees. “It must have been like a space-ship landing. To the average Mancunian it would have been out of this world.”

Hale and Rhead are founders of the Manchester-based Modernist Society which has a magazine, gallery and shop dedicated to all things modernist.

At the end of August, the society is branching out digitally with the launch of a free mobile app that guides people through the city’s 20th-century architecture. Soon they plan to expand the tours to Salford, Stockport and Trafford – and after that who knows.

On an actual physical tour of modernist Manchester, the passion of Hale and Rhead for the buildings was obvious.

The Express building on Great Ancoats Street is one of three built by Owen Williams for the Daily Express newspaper, the other two being in Glasgow and on Fleet Street in London.

Williams was a brilliant pioneer of concrete and the principal engineer on Wembley Stadium. He went on to play a key role in Spaghetti Junction and designed the bridges on the M1. The Express building in Manchester is regarded by many as his crowning achievement.

It’s a stunning building and one that tells a wider story about Manchester’s rich newspaper heritage.

“Manchester once printed more newspapers than anywhere outside Chicago,” said Rhead. “It served everywhere except London, so papers from Manchester would go to the north of Scotland as well as Bristol and Birmingham. All the newspapers had big offices here.”

The Manchester Guardian, now the Guardian, was on Cross Street and the Daily Mail was in a beautiful now demolished art deco rocket building on Deansgate.

In the eyes of Hale and Rhead, no newspaper building is as fine as the one designed by Williams that opened in 1939.

“It has all the basic principles of early modernism,” said Hale. “It’s functional, it’s using modern materials, it lacks decoration … it is one of the purest early modern buildings in the city.”

After sympathetic refurbishment the building today still looks glorious, seemingly thriving as a co-working space.

Which is more than can be said for the next building on the tour, the Co-operative Insurance Society (CIS) tower on Miller Street, which was completed in 1962 and was once the tallest office building in the UK outside London.

The top floors were all Mad Men-style wood panelled executive offices and there was art in abundance. The grand foyer has a Cornish granite floor, walls clad with Sicilian marble and a striking abstract fibreglass mural by the artist William Mitchell.

Today you can just about still see the mural through the dirty windows of the abandoned 118-metre-tall building. Outside, there’s a smell of urine. Near the entrance is the concrete Mitchell-designed water feature, which hasn’t featured water for decades.

Other buildings that appear on the app tour include the concrete Renold building at the University of Manchester, with its striking glass-sided staircase, and the former Kendals and House of Fraser department store building on Deansgate, a purpose-built “international functionalist” structure with more than 26,000 sq metres of retail space.

The free modernist tour app has been developed thanks to a grant from the Greater Manchester combined authority and Hale and Rhead have high hopes for it, including for other cities across the UK or beyond.

“People definitely share our passion for modernism,” said Hale. “We sell enough books and enough places on our physical walks to know there is a big interest so we’ll have to see how many people sign up.”

Hale and Rhead are in love with the “ambition and the optimism” of the people who designed and built Manchester’s modernist buildings and wish there was more of it today.

“Wherever you go there is always modernism, you will find it in every part of the world,” said Rhead. “It is called the international style for a reason.

“Today, I’d say there is a paucity of ambition, everything is commerce-led whereas this [pointing to the CIS Tower] is led by ideology. On the whole, modernists really did think that they were improving people’s lives be it through design or architecture or public art.”