When I got home, I looked it up and this is what I found:
'Indian BalsamImpatiens glandulifera
Introduced as a garden plant from the Himalayas in 1839, and naturalised along waterways and in waste places, this tall, stout-stemmed species grows 100-200 cm high. It is hairless and the stems reddish. There is no mistaking the rather orchid-like, mave, dangling flowers.
Flower: purplish pink, 2-5.4 cm, petals 5, forming a broad, lower lip and hood; sepals 3, lower forming a mauve, spurred bag.
Flower arrangement: long-stalked racemes arising from leaf-axils.
Flowering time: July - October.
Leaf: opposite or in threes, 5-18 cm long, elliptic, toothed; reddish glands along basal margins.
Fruit: capsule, club-shaped, opening by 5 valves, which spring into coils, shooting out seeds.'
Information from 'Illustrated Guide to Wild Flowers' by Stephen Blackmore
The Indian Balsam flowers were just behind me as I stood admiring the lock keeper's cottage below.
You can see more of my walk along the towpath here.