Downy mildew which thrives in damp conditions, and causes angular patches on the leaves between the veins. The fungus itself is more often visible on the underside of the leaves. Badly infected foliage will turn brown and die back when conditions become drier.
Try a Fungicide.

Ours has the same on some leaves also one of the Spireas affected.

Hello , thank you both , yes i thought as much , i do have a fungicide so will give that a go , one large bush is looking somewhat better but we havent had any rain for a few days !!! . doesn't seem to have affected anything else so fingers crossed .
All the best Lucy .

Hi Meadowland, Hopefully your Spireas start to recover too , been a funny spring for a lot despite all the Rain !

Thank you Heronbay...hopefully it works...the Buddleja here is a compact version with red flowers. Newly planted last year.