Tiny pale blue flower carpeting a lawn.
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I think I'll kick myself when someone identifies it - I really can't think what that is at all, sorry. Reminds me of Tritelia, but that's much bigger, and I think has 6 petals not 5.
It was on a path, so I suspect someone came out for a walk in muddy boots and brought it from their garden.
I've a nasty feeling we'll all be seeing more of it.
[Later correction: it was Sand Spurrey, not Pratia]
Interesting. I think I have come across it somewhere before, but not enough to feel bad about not recognising it!
Just looked it up in Stace, and Pratia angulata is mentioned, as 'becoming established in damp lawns, from NZ'.
This page http://www.paghat.com/pratia2.html suggests that the two are not the same plant, although some suggest that they are.
Finding it in heath grasslands is not so good. The website above says it's edible - perhaps we should eat it!
I'll take this as a useful reminder that I really should know better than to trust the names that most nurseries and gardeners use.
Lat/Lng: 51.528, -0.478
OS grid ref: TQ056821