Drought? Need more info, like how long have you had the plant, is it in a pot or the ground, is it in full sun?

The plant is approximately 8yrs old and is watered regularly. It is planted in the ground against a south east facing wall and gets the sun till mid afternoon. There are a few 2nd flush flowers on it, and it is growing vigorously apart from this area of dieback.
Thank you

Ah, it was not clear from your question that only one area is suffering in this way. Suggest you inspect closely to see whether the shrivelled leaves are all (hopefully) coming off one main branch - if so, trace it back to point of origin at the base - there maybe damage, infection or infestation to that particular part. Remove the whole part if necessary.

Have done that now, fingers crossed it was only that branch affected. No sign of the problems you mentioned! Thank you, your help is much appreciated