Continuing, like London’s flow, west to east and down to the sea, your route is well-marked. It’s scuffed with coastguard emergency notices. Each step leads you away from where that help’s mustered; where palaces dip toes into Greenwich’s more genteel, upstream waters.

Your path hugs the river, caught between an archaeologist’s ancient, silted dream and the planners’ square-framed spectacles. Half-buildings grow up beside patches of bramble, derelict storehouses and, holding old secrets the tightest, swirling rip-currents.

At Bazalgette’s monumental pumphouse the trail narrows. Hoarding hides the riverside view and iron railings bar access. They’ve reduced its statement of domineering Victorian virtues – a temple, an institute of learning and a beacon to cure the metropolis of sewage – to an apology.

Its measure follows. Guarded by taller, stronger fences is the vast, replacement, privately-owned works. The next huge complex is devoted to the disposal of other urban waste. The city’s topography and its prejudices have combined profitably to send its flotsam and jetsam sliding away here, out of the general sight.

With the path twisting around angled quays containing their skeletons, rusting gantries appear. Forlorn testament to the frantic comings-and-goings of the overseas trade they once controlled, they’ve been left to rot and, only afterwards, had their ‘dangerous structure’ warnings nailed on.

Erith completes this stage. A broken-down train just beyond the station on the upline gathers heaviness, like some beached whale. It blocks all London-bound services. At least you have a kind of map to find your way: back or on.