as far as i can tell there is Ribena really light, extra light & light all of which seem to be pretty much the same drink just rebranded and with the formula slightly tweaked.
The Ribena light is the current version thats out and i've been treating it as free as it's a sugar free cordial type drink, maybe it should be 1/2 syn but if it's an occasional drink (i think they only do the individual pre-mixed in "light") i wouldn't be too worried about 1/2 syn either way.