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It’s not mildew, I know what that looks like.
I’m pretty sure now that it is some kind of virus, having read the RHS site.
I don’t think it’s down to any of those, chip, apart from maybe frost damage but I’d be surprised. The leaves, and indeed the whole plant, are all very healthy looking, not dying, shrivelling or dropping off, just turning variegated.
I think we’ll see what happens as time goes on and the roses flower, and take it from there.
Thanks all, much appreciated.