A leaf mine to currently look out for on St John Worts, perhaps more easily on the cultivated varieties in gardens or the sub-shrub tutsan Hypericum androsaemum in woodland, is Ectoedemia septembrella. Hypericums of course are familiar shrubs in parks and gardens, with deep yellow flowers in summer.
The photo below was taken in my garden this evening (29/7) on a plant that I grew from cuttings taken from a Hypericum in a park on the outskirts of Ammanford almost two years (20.8.19). I had recorded septembrella on that occasion too. It makes me wonder if the micro-moth somehow `hitch-hiked` with the cuttings that I brought home or did it arrive independently? I suspect the latter, as this species has been noted elsewhere in Llanelli on planted Hypericum shrubs and, when taking my original cuttings, I would have removed most of the leaves.