On page 11 there is a table detailing certain criteria if it were to occur and the results/actions taken to remedy the potential problem.
While not quite specific, it seems as though suppliers have actions prepared and in place in the case of an incident of modern slavery. Russell Investments seems to have faith in them.
"For this second reporting year, suppliers seem largely in the process of bedding down governance and procedural processes implemented in the first reporting period. We consider that our suppliers are developing appropriately at this stage." - Page 11