Whatever it is, its not any kind of Pieris... it might be a Sambucus racemosa aurea, but hard to be sure - is this all there is of it? How long have you had it?

I don't know how old it is, as it was in the garden when we moved here 2 years ago It is about 8 feet tall and 5 feet wide and has quite a substantial tree trunk.

can you add a photo of the whole plant, so we can see growth habit, in an attempt to ID it properly? By the way, I'm not seeing mildew in the photos you've posted, though I can see discoloration of some sort on the leaves - anything interesting beneath the affected leaves?