Hi, it could be Leycesteria formosa (pheasant berry) as it looks like a photo I posted last year asking why the leaves had grown like this as they are different to the original plant, you could have a look at my question and see what you think, they do self seed quite freely too :-)

It depends if you want to keep it or not. Plants are only weeds if you don't want them.

Should also have said it's a shrub that can grow quite big about 6ft

Thank you Daylily and Rosierose for your responses.
Nanny hil ?