Confidence: It's likely to be this, but I can't be certain.
Notes: Not Potentilla reptans, because:
- the flowers are borne on a stem that's leafy above the base;
- the leaves on that stem are trifoliate, not 5-foliate
- the petals are barely overlapping.
My best guess is the rather less common P. anglica (Trailing Tormentil). If it were, one would expect to see a mixture of flowers with 4 parts and 5 parts.
But there are several hybrids to confuse the issue, and one would need to check the whole plant and also its fertility (most easily done by seeing whether most of the achenes are aborted on a fruit).
P. anglica hasn't been recorded before on Roundhill but it's perfectly suitable habitat, and there are records from the adjacent bits of Beaulieu Heath.