It 'looks' like a rather unhappy vetch, one of the pea family and it can be a plant or weed as you choose. I don't think that it is one that I would keep.

Tufted vetch ?

Could it be harmful to other plants?

No not harmful, just a weed.

Hi Treetop, your neighbour is right, it is a plant, but it's also a weed, a weed is just a plant that's growing where it isn't wanted, Derek.

I have it as a cherished plant in my garden and it doesn't cause problems except it seeds freely.

Thanks folks, I'm feeling pretty guilty now, having taken it out.