It looks like Leycesteria Formosa.

Yes, that's the one. It self seeds furiously so needs thinking about before getting one. On the other hand, it's very popular with birds, especially blackbirds.

Thank you both! I've googled some images and it does indeed look to be that one. It's a self seeded plant, my friend didn't know what it was, so she's really pleased now I've been able to tell her.