Identify this insect pupae/larvae ?

Not sure if this is an obvious one but I can't seem to find what species these are specifiaclly, this is my Nan's garden and she's never seen them before, we're in the north west if that helps narrow it down and we've had warm weather of 20 degrees average the past couple of weeks. They appear to have six small legs also and are on the tops and under side of the leaves, unsure if they're linked to the destruction of the leaves of the plant as well but the leaves on the lower sections are near completely gone. Any help would be appreciated, would like to know if they're an important species, butterflies of any kind etc. Or if they're a pest gearing up to destroy the garden.