I wonder if you remember this unidentified plant which was included in a blog in August titled 'Garden Mixture, August, still'?
I wrote to the Royal Horticultural Society after ringing in the first place. I was asked to send photos and a sample of the plant material. Gillian had just dug them up so I recovered some from the compost heap and sent those. Yesterday I had an email identifying it.
The RHS wrote:'Our Botanist says the sample and photographs you sent for identification appears to be Ambrosia artemesiifolia, a somewhat uncommon annual weed in Britain, most probably from bird-seed or possibly an impurity amongst garden seeds. Although quite a serious agricultural weed in parts of the US, it seldom becomes a problem in Britain.'
It's not from garden seed (didn't scatter any this year) so must be something in the bird seed. I hope it doesn't recur next year though we'll be able to hoick it out directly it appears! They also reminded me that their ID service is for Members Only and this was a one-off - fair enough - but there's no point in me joining now as I don't really garden, just direct Gillian... It's quite a lot of money for no benefit as far as I'm concerned. But I will send a small cheque in appreciation.