Last month England released a new report detailing a country-wide audit of non-native plants, animals, fungi and microbes. As noted by NFU Countryside, the report concludes that there are more than 2700 introduced species and hybrids in England, the majority of which (73%) are plants. Of the 2700, about 1400 species are considered to be established enough to warrant concern, and a mere 19 were listed as "having strongly negative environmental impacts." You can download the full report (.pdf), which includes a detailed description of the criteria and categorizations used, from English Nature. I commend the authors for their tremendous effort, though I am a bit disappointed that their pathway analysis assumes only a single vector of introduction for each species.
Thanks to a member of the ALIENS-L listserver for posting about the report.