Co-indidentally there was a feature on Japenese Knotweed on The One Show just after I posted this thread.
Looking at the footage, these seem to be different stages. A young version where the stems are very red and slim (like the plant in my garden) and then it starts to appear bamboo like.

Confused, yes.

It is a very small patch, but we are planning on building extension on that land and need to know if there is anything in there that will potentially damage our plans/building.

Thank-you for looking