Hi all.

Over the years my neighbours have had some Knotweed appear, so, when I noticed some tall, thick, reddish plants shoot up suddenly I assumed it was the same.
However, on closer inspection, it seems that the stems have large thorns on them, and seem to have some reddish fruit(?) growing off them.

Oddly, these seem to have appeared and grown-up several feet over the past couple weeks, despite the freezing temperatures here.

Any thoughts on what it is?

Thanks,
Paul

Looks like a type of wild rose, maybe R canina.

kniphofia is correct...it is a Rose.
Definitely not Japanese knotweed.

I suspect that earlier on it was covered by other leaves/weeds/vegetation and as winter has progressed they have rotted and died away. Leaving the Rose branches with the thorns on looking more obvious.
In the cold weather your Roses will not have grown.
In spring with warmer days you will see new shoots growing followed by flowers. After the flowers you get the seed pods or rose hips.

Pics below show Japanese knotweed...new leaves and a forest of stems /close up of leaves etc.

I agree, it looks like some type of Briar Rose. Here's our database entry for the one Sue suggested: Briar Rose (Rosa canina) and a couple of photos for comparison:

That's awesome, thanks all.