Tourists know it as the building next to Harry Potter’s Diagon Alley. Office workers see it as a prime address for insurance giants. But for Hertshten Properties, it is a big opportunity.

The family-run investment group, part of the global Hertshten Group trading empire, signed construction company Mace last month to move ahead with its bold new office tower in London.

What began as the relatively straightforward redevelopment of 85 Gracechurch Street took an unexpected turn when archaeologists uncovered the remains of Roman basilica foundations beneath the site.