the stalks are hollow and im not sure if this is a flower or a triffid

I would think this is Leycesteria formosa which is often a present from the birds. Also called Pheasant's berry in our neck of the woods, or Himalayan honeysuckle. A loose shrub growing up to 2.5 metres. If you look it up and compare leaves and stems.

Or Himalayan Balsam. It's often seen on canal & river banks in my area

it is lycesteria Formosa common names are as honeysuckle says.
it is not Himalayan balsam, the leaf is wrong and the flower buds don't look like this either.

it will have been seed dropped by a bird. it makes a nice mid sized shrub that is easy to manage. so if you or your dad want it replant where it will have space.

I agree Leycesteria formosa. Lovely long racemes of red flowers then berries. That Himalayan balsam is a menace along waterways round here.

Himalayan balsam is also found away from water, as long as the soil is quite moist. it is a lovely plant but sadly no real checks on its growth. my youngest girl spends a lot of her time pulling it from the woods on several of the reserves she has been working at.

I used to grow it for using at school. the stems are really easy to dissect out and of course its seed pods explode their seed, so really good for that aspect. it never really got to be a problem in my garden as it was too dry really.