The BBC reports that the Rosser's sac spider, thought to be extinct in the UK, has been found alive and well at Chippenham Fen in Cambridgeshire after 10 years without being seen. For an alternative and more amusing take on the story, try io9's version!
I've struggled to find a page on Rossers' sac spiders that delivers the full low down on them, but I get the impression they're really not that big, though probably possess a nasty bite for their size.