The greatest find of Roman antiquities in London, just by Mansion House Tube station, owes its survival to a humble stream – the River Walbrook. The brook doesn’t run far – from Finsbury, on the north edge of the City of London, before emptying into the Thames by Cannon Street railway station.

It has been used as a rubbish dump, covered over and thoroughly ignored for thousands of years but, still, Old Man Walbrook just keeps rolling away. And it is thanks to the Walbrook that some 10,000 objects from Londinium – including an unprecedented haul of 250 leather shoes, pewter dinner plates and dozens of wooden writing tablets – have survived in better condition than anywhere else in the Roman Empire. In the few weeks left of excavation, even more finds are bound to turn up.

What has emerged is a thickly packed development of long, strip buildings, with ovens, kilns and a mill, nestling up against a Roman temple; a bustling residential, religious and industrial mix. The teeming excavation site – in the shadow of Bloomberg Place, where the European headquarters of the financial information empire are due to be completed by 2016 – has already been dubbed the Pompeii of the North.

That may be a bit of an exaggeration. But the preservation of the Bloomberg Place artefacts is as good as in the Neapolitan seaside town, thanks to the waterlogged London mud. It didn’t just preserve wood and leather; it even kept metal intact – with no access to oxygen, the copper brooches, lead plaques of prancing bulls and copper phallus decorations for horse harnesses, couldn’t rust.