Thanks, Mike & leifus
474 isn't at all bad, and a lot of my plants were found in pretty unpromising areas of urban wasteland so if you live in or near a town you ought to be OK to add quite a few more this year.
As for my quest, it's not begun on the best of notes - so far I'm on minus one for the season, having misidentified Stinking Hellebore as Green Hellebore at Slimbridge last month
That ought to be one I can get back easily enough later in the spring, though - I've been told they're native to the Avon Gorge, which is one of my local patches.