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Unfortunately Vinca is rather variable and flower size is not always a certain indicator of species. However there is an easy method. V. major has hairs along the edges of the leaves and on the calyx (and perhaps elsewhere). V. minor does not. This specimen looks to be hairless, but it's a little difficult to be certain.
BSBI VC Recorder
It seems to often be the case about flower size.
I don't think the leaf margins were hairy but I will know to check and take a close up next time.
Lat/Lng: 53.6918, -1.8355
OS grid ref: SE109217