Please could you add a photo to show the extent of the problem? Is it in a pot or in the ground? Is it an established bush or a new one?

This has been a bad year for Hydrangeas. They like a lot of water and it has been a very dry summer. I watered mine every day but the flower heads are not as big as normal.
It should be fine next year but remember to water it.

As requested, a photo is really rather essential - its hard to answer your questions without seeing the plant, or having more information.

Most likely water. Hydrangeas love their water - lots and lots of water. Esp in this heat.

As you haven't added a photo can you say whether the affected leaves are all at the tips of the branches or scattered randomly around the shrub? Also
please answer the q. re pot or ground? You can write a reply underneath the last answer, ie in the comments box.

I decided to take the affected leaves off the plant anyway, so a photo would probably not have helped. The affected leaves were scattered around the plant, which is planted in the ground. Looking back, I did not water it enough and that could be the problem. It's a learning process! Thankyou for all your comments.