Well. Until now anyway.
A few weeks ago I noticed them slightly wilt. I thought they needed water, which is a bit odd because they've never needed a drop of extra water. Turns out they definitely didn't need water, they needed treatment for powdery mildew. This is what they looked like at first, and what the one in the sunnier position still looks like:
The plant in the shadier spot was so far gone that I took a punt and chopped all the leaves off - they were dying so quickly anyway. When it's that far along, fungicide sprays can only do so much. So I hacked it back to this and doused the stumps in fungicide. Let's hope its tough nature lets it bounce back.
Even if they die, I've decided I'll replace them with statice. Next year in the summer I'll be sure to cut out all the dead foliage and I'll try using a milk spray a few times in the summer. Milk spray puts "good" critters on the leaves that don't leave room for "bad" mildew. It won't work after the mildew sets in but it could head it off next year.