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Apologies for the late reply Troublesome knees. Hope that you get this in time. The City Pride is right on the route. It is about 3 mins walk from Heron Quays DLR or about 5 mins walk from Canary Wharf tube. They have a balcony that overlooks the route too. The beauty of the location is that runners go past twice. It is advisable to get there early though. See link below.
I am not sure that pedestrians are allowed in the Rotherhithe tunnel but even if they are I wouldn't advise it. Too many fumes.
My apologies Tricky Dicky,
I have just found out that pedestrians are allowed in the tunnel and that a group of 20 hardy souls brave the journey every day. I wonder what their lungs are like.