I'd get the platy out of there and isolated. I know you're cycling your QT
tank, but to be honest, you don't need to. For future reference, you can keep an extra filter sponge in the main tank filter, and use that in the QT
as needed. And keep in mind, most meds will kill the nitrifying bacteria anyway; its a lot of work cycling a tank for nothing! I'd do a major water change on the QT
and get the lil bugger in there.
I'd continue to treat for ich, but like I said, I suspect other issues, which is why I'd isolate him. Its a prob if there is something else since your in the UK; antibiotics aren't sold over the counter like they are here for fish. You may need to call a vet and see if they'll give you a prescription for erythromycin or tetracycline just in case. It may not be necessary tho; just want you to be prepared.
aka Cycling Guru and the Ich Slayer
*glares at Terry and QTOFFER*
Card carrying member of FTAS & GCAS.