They are really thirsty plants needing copious mounts of water andits been so hot and dry,
I think they are shallow rooted, when the ground is wet through add a mulch to conserve the wet.
Water regularly including the leaves but not in high sun, you could water morning and evening if very hotand dry
oh I'd give it a feed but not until next spring.....too late this year

If you rub the blotches do you get coppery red dust on your hands? If so you have rust!
Found this treatment (for those who don't like fungicide) - "Rust, black spot and powdery mildew can treated without toxic chemicals by dissolving two 325 milligram uncoated aspirin tablets in 1 quart of water. Once dissolved, put the mixture in a clean garden sprayer and liberally coat the hydrangea with the liquid. A second treatment may be required seven days after the initial application to completely control the rust."
Cut out and burn the infected leaves if it is rust.

I wish you could add questions to favourites.

No it's not lack of water as they have been well watered each day.

I'll try the rust test tomorrow as it is dark now.

Thanks to you both for your answers.

Stera, I use a tablet pc and although its not saved on Goy can save the page for offline reading

as to the red leaves I recall a question on gardeners question time R4, this was about cherry trees......they put it down to the trees reducing their leaves because of the weather, I notice my silver birches have some leaves yellowing, all to do with the amount of water the roots can take up and distribute through the plant versus the amount its losing through the leaves........

My cherry is doing the same - autumn coming early!

Well I suppose spring did.......