Being in a park, along a route commonly used to access the reserve, suggests that it was spread by a walker. Somebody has clearly tried to arrange some protection for it. I walked past there yesterday without seeing it.
Since this sighting, which has been relocated a few meters away to a 'no mow' zone, 2 additional colonies have been discovered, one of 2 happily already in a projected no mow zone, and another, near the end of the slippery dip, discovered by Chris Mobbs, to have had 9 (!!) individuals in an area c 1m square, some of them plausibly yearlings, others that must have been around considerably longer. These are being relocated. But the evidence that they can survive a certain amount of attention from the (not extremely assiduous) park mowing is perhaps interesting. Maybe there are more colonies there and in similar places!