just realised I can use the 'always show mouseover state' option for 'behaviour on mobile devices' to sort this but I am also using 'zoom in' so the thumbs on mobile are slightly zoomed in which is fine but do show the descriptions at all times which is good.
Would be nice to have the description show always on mobile but the 'zoom in' effect only applies when you touch the thumbnail but no worries, i'm happy with this solution.