Confidence: It's likely to be this, but I can't be certain.
Notes: Size about 10mm. With tail. Feeding on Fuscia in garden. Elephant Hawk moths do feed on this, as well as Rosebay Willowherb. Was about to throw away a broken twig of fuscia, when I thought I'd pop it in a bottle to root. Later noticed holes in leaves and droppings on window sill! Found 4 of these larvae on the one small twig! Hoping they turn out to be the Elephant Hawk moth.