Yes, they are hoverfly larvae and I have always attributed these to the genus Epistrophe, but have never gone beyond. There is a colour guide to the larvae of hoverflies (see here: http://www.nhbs.com/colour_guide_to_hoverfly_larvae_tefno_7239.html) not currently on my shelve, but one I have been meaning to get - I will order it this weekend.
Older stages will be somewhat flatter. Also colour may vary from pale brownish green to clear green. The ones on your image I gather are nearly full grown. Using aphids as comparisson for size may be deceptive: small larvae may appear gigantic in a colony of small aphids but dwarfs in a colony of large aphids.