Hairy Violet

Walking back from the butcher's I spotted the first violet of the year growing by the roadside. I assumed it was either Common Dog-violet or Early Dog-violet but close inspection suggests that it's Hairy Violet or Viola hirta. 
There are several species of violet in the UK and telling them apart can be tricky, but a good place to start is the presence of hairs on the flower stalk: either Hairy Violet or Sweet Violet. Then it comes down to looking at the structure of the sepals and spur. 
My favourite is Sweet Violet but it's not as common as the others round these parts. When we first moved into our house there was a good patch of Sweet Violet growing down in the orchard but it's no longer there.