Confidence: It's likely to be this, but I can't be certain.
Notes: A distinctive larva for a geometrid of its size, less than 2cm. It has a 'frilly' undercarriage and sparse short hairs. Two images of 15mm Light Emerald larva are available in ukleps and this is pretty much identical. Found on hazel leaves. The adult is commonly encountered at the site earlier in the summer.