We think these are the sawfly larvae having a good old munch on the leaves - they practically stripped the plant bare.
According to the BBC website, sawfly larvae skeletonise (good word!) leaves or eat them to transparency, leaving just the veins remaining. If disturbed, they curl up into an 'S' shape (just as they are doing here).
They are probably under-recorded as a species and come in different groups, often defined by their menu of choice: the gooseberry sawfly, the turnip sawfly; the rose slug sawfly.
