GeoWizard: No Roads Mission

My most ambitious/mental urban adventure to date draws to a reluctant close, but I've saved all the best content till last. My ticket to the southern edge of Greater London is now via the Ravensbourne river, a fascinating but sometimes filthy semi-subterranean tributary of the Thames. My decision to leave it temporarily results in one of the most exhausting shortcuts ever taken, and a subtle flaw in my route late on that leads to one of the most wretched ordeals me or my uncle Ed have ever experienced. Will we make it out of the sewers and complete the mission with no deviations? or will my luck finally run out...