Interestingly, the BSBI have posted an article (
http://www.bsbi.org.uk/html/alien_invaders_.html) by a couple of members disputing the extent to which alien invasions are a cause- rather than a symptom- of ecological problems (in a similar vain to
Red Squirrels v. greys v. forest-degradation arguments).
Nonetheless it seems worthwhile to take part in Plantlife's survey; whether you view the aliens as a destructive force in their own right, or as an indicator of other problems, the data is still useful.