Thanks Rick. It would have been my fourth of my hybrid crosses to flower (I've had two Asiatic and one Aurelian cross flower thus far - thankfully the Aurelian seedlings show no symptoms).
You raise an excellent point about the risk posed by the virus(es), especially if 'Pink Giant' was the origin. I'm definitely about to dispose of Plant 2 (not fussed by that one) and am strongly contemplating getting rid of that pot of seedlings containing Plant 1 (unfortunately I had a closer look at the other two smaller seedlings from the same batch and they also appear to have similar markings, though not as obvious). I think I'd be better off just starting another batch of seeds for this cross, unfortunately.
