I agree with Richard on the Ragwort - We have a bit of it in and around our wood and the cinnabar moth caterpillars like it.
See this link:
http://www.plantpress.com/wildlife/o278-cinnabarmoth.php
Having said this there is legislation governing Ragwort and a few other 'weeds' (what a surprise!) and you can find out more here:
http://www.defra.gov.uk/rural/horses/topics/ragwort.htm
This site has links to lots of advice and codes of practice - most of which the local Councils ignore 'cos they don't have the money to clear the 'weeds' from their land let alone enforce control on others!
I've heard that the definition of a 'weed' is a plant growing in the wrong place
Greyman