I saw this little plant growing out of mossy stones and stumps all around Heathercombe. I was trying to think which rare native flora it might be. Then the penny dropped- it's Pink Purslane (Claytonia sibirica), a plant from North America and Siberia that has naturalised in parts of the UK after being introduced as a garden plant.
 I have it in my own garden! But I didn't recognise it at first because it looks so thoroughly at home in the surroundings, much more vigorous, larger flowers and pinker in fact. Some plants like it tough; anything with the word sibirica in it is invariably very hardy. Dartmoor obviously suits it more than South London.