Looks like a Clematis or Passion flower, but someone will know.

Thank you. It's not really very pretty though. It's a bit of a beast!

Clematis, either vitalba or fargesii.

I thank you very much Owdboggy. Thank goodness I didn't pull it all out !

Both of them are total thugs, especially vitalba. C. vitalba is the native one with the huge 'old man's beard' seed heads in autumn. It spreads like wild fire both from suckers and seed.
C. fargesii is also rampant. We have it and it has covered a huge clump of Bamboo and the Gazebo next to it. We cut it down to ground level after flowering and before the seeds are ripe to try to stop it spreading even more.
I doubt that you could pull either of them out altogether to be brutally honest.
Looking again at the image I would err on the side of C. vitalba.

Thank you for taking the trouble to give me all that information. Now I can tell my neighbour that it was beneficial to the garden. When I tried looking it up online I thought it was Hogweed ! Thanks again Owdboggy

Things are only beneficial to the garden when they are wanted and you seem to feel this one isn't - just because its called Clematis doesn't mean you have to have it. It will go mad if you let it.

I think it's lovely! But a weed is just a plant in the wrong place.