Kew no longer help in ID ' ing plants unless you cross their palms with silver as they are a private business.
They were very kind and gave me the addy of the natural history museum , hidden away on their website in nature plus is an identification section for plants , trees , flora & fauna etc.
The site is difficult to access on paupers village speeds but may well work better on a high speed internet connection .
Here is the address
www.nhm.ac.uk then go into the nature plus and sign up to them .
The plant is called ..... Xanthium spinosum or cockbur it is quite common all around the world inc UK , is an annual plant , toxic to animals & is usually found in disturbed soil.
Alison reckons our garden is very disturbed just like its owner .
I suspect the seed has lain dormant for years as the stock pile of earth used to rebuild the garden has been there for many years going back to around 1970
If you interested bung " Xanthium spinosum " into wikki for a wee bit more info