find this about the Ashdown Forest(
Ashdown Forest Tourism Association - Local Information, Places to Visit and News & Events from the Heart of the High Weald)
Quote:
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Marsh Gentian, which is becoming increasingly rare although there are still about a dozen colonies to be seen, flowering from June to October.
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So given that's a dozen colonies across about 20000 acres (ish) it's not surprising they're a bit difficult to find.