Wild Pansy (Viola tricolor), which as the name suggests has flowers of purple, yellow and white, grew quite commonly across East Anglia as a plant of open short-sward grassland on light sandy soils. When it came into contact with the little yellow and white annual field pansy, Viola arvensis, they hybridised. The progeny is this plant, Viola x contempta. It gets its large multi-coloured flowers from the V. tricolor parent and the purple colour on the arch of the flower stalk from V. arvensis.
If you want to find it, go to an area with sandy soils and check out the arable field margins – you are looking for a little pansy with blue or purple splotches on the petals and petals about as long as the sepals.