@Dougal Hi Dougal. It's most likely heat stress. The rose bush turns yellow and drops its foliage in an effort to cool down It will lose the yellow leaves. A good water (prob getting that today) and a handful of rose fertiliser will help. A mulch on top of the soil with composted horse manure will help but not essential. Watering most important.
Good morning GTers - things are starting to happen at Chez Matthews!! 😃