Upon further consideration, several things have occurred to me.

First, this must be one heck of a sweet beverage if it must be thinned
out before fermentation. Various sources online describe Ribena as a
beverage, but your recipe describes it as a syrup. Are there different
varieties of this product?

Second, and more relevant to your query, perhaps they have changed the
formulation of it. Are you certain that it only contains SO2? Even a
small amount of other preservatives would prevent fermentation. Look
for "Sorbic acid" or "Sorbate" on the label. If you find either, I very
much doubt you'll ever get the yeast started.

Regards,

Greg G.