Hello Iain,
The option -roi_labels takes a file not a value. In your command, dataset column (roi-SUMA-1.closest.1D.dset[0] ) should specify the indices of the nodes (node IDs) forming the ROI you want to grow. If these indices come from multiple ROIs, the option -roi_labels is used to specify which ROI label (integer label) each node in -roi_nodes belongs to.
Be sure to check your results visually with SUMA.
cheers,
ziad