Longhorn Moth (Nemophora degeerella)
To see these in the sunshine sitting on a leaf like this is not uncommon as they are distributed across the country in areas of deciduous or mixed woodland.
Most frequently, however, you see them in woodland clearings in May and June dancing up and down (not dissimilar to a mayfly). They are much harder to identify then and you cannot see their lovely colouring.
With the sun on them they are wonderful combination of gold on a metallic green back ground.