well you could spray them with a suitable fungicide before any symptoms develop. clear any fallen leaves from last year if there are any still around the plants.
do they have plenty of air flow around them?

Hi Sbg they did have plenty of room last year, maybe I should wait and see then as they were quite large when I bought them, maybe they already had it, is it something that can come and go?

Powdery mildew comes and goes, its not a permanent condition lasting from one year to the next. Its often caused by dryness at the roots in the plant that's affected, so ensuring that doesn't happen will help, but if it does happen, use the milk remedy, much easier and cheaper, you just need your own spray bottle. Mix 1 part cows milk to 9 parts water and spray thoroughly, till its dripping off. I stick to skim milk because its less likely to smell than full fat does...

That's interesting thank you Bamboo, thanks for your reply too Sbg :-)