Plantlife have replied that they think this is Large-leaved Geum, Geum macrophyllum. A plant of the USA and first found in the wild as a garden escape in 1908 and now found in several locations throughout the UK.

Interesting, I thought the forming seedheads looked rather Geum-like and wondered if it was a hybrid avens with more wood than water. But it didn't quite fit so I was still mulling. One to watch out for.