I found a pale yellow stippled effect on many of the leaves on my blackberry plants, and some small leaping insects under the leaves, can you advise please?

The problem is leaf hoppers.

These wingless nymphs feed under the leaves on the sap, resulting in the bleached spots.

As they mature, their wings gradually develop and they become adult insects.

When disturbed they make the characteristic short leaping flights, hence their name.

Spray under the leaves with a suitable insecticide.