Well its a leaf spot of some kind - have you checked the undersides of the leaves and the stems of the plant for infestation of any kind? This looks most like fungal leaf spot, except I can't see any tiny raised fruiting bodies in the centre, though it's possible those have died off since the plant's been uncovered and all that's left are the tiny pin sized spots in the centre of a couple of those large spots. I'd give the plant a good prune, getting rid of most of those affected leaves - you could try a fungal spray, but its a bit hit and miss as to whether the fungicide in the one you choose will treat the fungus that's caused this. It may have been something overwintering on the plant underneath the fleece, particularly with any build of moisture on the leaves. Leaf spot of any kind doesn't usually kill the host plant, just makes it look unsightly.