I can advise that following a recent walk from the Boulters lock access path to Ray Mill Island, and then across the footbridge which links Ray Mill Island to Glen Island, that Glen Island is a mixture of public open space and private housing. The dotted red route seems to be an accurate mapping of the footpath that leads the the roads that service the housing which provide access to Bath road.  The path appears to be open to cycling and consists of a well maintained compacted cinder surface.