our burford holly shrubs have developed rosette clustered leaves

Question

These clustered leaves are on many of the plants. They are small shrivelled and some have turned dark.

Answer 1

It sounds like your holly has developed a condition called witches broom. In warmer climates like yours, it is typically caused by Sphaeropsis tumefaciens.

This article on it should help:
https://www.gardeningknowhow.com/ornamental/trees/tgen/witches-broom-disease.htm