The brown areas suggest dead tissue rather than the remains of the outer layers of the leaf following on from a leaf miner. A possible leaf miner candidate is Incurvaria pectinea, but it should be possible to see either the larvae in the mines or the cut outs left in the leaf when the larva has dropped to the ground. There is a range of pictures of this miner here:
Could one of the case-bearing moth larvae in genus Coleophora also be a candidate? See for instance the leaf shown here:
Entomologist and biological recorder
Could be a coleophora, but I'm nort sure. Serratella feeds from the underside of the leaf. The picture shows the upperside of the leaf, but it should still be possible to see the puncture marks made by the larva. These would show as small holes, roughly in the centre of each blotch. I think the only holes visible are 2 small tears near the bottom of the picture. I don't know about mites either.
Looking at the various possibilities, the closest I can find is I. pectinea at www.leafmines.co.uk/html/Lepidoptera/I.pectinea.htm
It has the multiple mines, starting as small brown patches, then becoming scattered holes. Not much else seems to look like this, though I wouldn't know if there are any potential mite species. Anyhow, I'll post it as a tentative i.d.
Have a look also at the picture here:
taken on the 13 June, slightly earlier than the iSpot pic. Given that there could be bit of difference from year to year in how fast the larvae develop, in a southern England location pectinea should have certainly have formed cut- outs by the second half of June.
A note that I put on the file itself suggested a mite might have caused this, any thoughts on this?
Main thought from me is that I don't know much about mites and tend to overlook them when considering these sort of questions!
Lat/Lng: 51.955707, -0.656901
OS grid ref: SP923294