Hi Sarah, it's hard to tell from the photo. Guignardia aesculi is a fungus that causes brown blotches with a yellow margin - that may be the case here. If you could find a fallen leaf you may be able to see signs of the rounder brown blotches of ohridella. Sorry I can't be sure.
Thanks Ina. I wasn't sure. I just thought the patches looked a bit regular and were in between the veins so could have been the leaf miner. I'll check again when I'm back.