Check the bush thoroughly, it might be Berberis sawfly. If you've noticed any flies around it, that's an indicator, but you might find caterpillars or larvae on the bush itself. Overwinters in the soil, so if it is that, control is the best you can hope for - spray thoroughly with Ultimate Bug Killer (provado) and cultivate the soil a couple of times during winter in hopes that any overwintering larvae will be turned onto the top of the soil and the birds might eat them.

One way of testing is to get an umbrella, open it, hold it under the bush and give the bush a good shaking - if there's any larvae present, some should fall into the open umbrella.

No caterpillars - no insects at all, no eaten leaves, just curled up ones, some now falling off.
There is Ivy growing behind it - could this be a problem?
I have cut out a few of the stems [as advised by AT in Gardens World] possibly not enough as it is rather tangled but am afraid of ruining it by being too severe.
Coincidently it only had a very few flowers this Spring, unlike last year.
It is 3 years old.

Check the woodwork - if something happened to the main branch the sideshoots might get sacrificed. Has anything snapped or are there lesions further back into the plant?

Sorry to take so long to get back - not berberis sawfly then. Do as Teadrinker suggests, look for damage, but I'd ask whether there's a tree stump or anything woody that's rotting in the vicinity - the only other time I've seen these symptoms it turned out to be honey fungus, though we had to wait till September when we saw the actual mushrooms at the base on a nearby tree which was also ailing.

Now it has defoliated much more - I think it must be Honey fungus [there are a few mushrooms near the base]. The problem is that what I think may be the cause, [a fir tree that was de-branched but trunk left, now has bark all stripped off] is in a neighbours garden behind a large fence.
What can I do if this is going to spread to the rest of my plants?

Honey fungus produces its mushrooms in September, usually, not at this time of year, and they are, indeed, honey coloured.

If it is honey fungus, it comes in different strains, and some are more virulent than others. I ended up removing shrubs in an area and replanting with things that were not particularly susceptible to honey fungus (hebes, pittosporum for instance) and plants classed as grasses or perennials, which also aren't affected (bamboo, phormium). The newly planted stuff is all fine, but the rose 9 feet away has now succumbed to something, which we assume is the advance of honey fungus. There is no effective treatment.

Oh dear that means I may lose several shrubs then, also My Acer [planted last Autumn has now started dying although no where near these others -could it be because of all this intense heat and sunshine? Can I transplant it?

My 8/9 year old oak has masses of Knopper galls, never had them before so this must be an awfully good year for the flies.
How can I stop this happening next year - should I spray it with something before acorn setting time?

Knopper galls are caused by a gall wasp - they don't cause any harm to the tree, other than possibly decreasing the amount of viable 'seed', i.e., acorns. You may find lots one year and none or very few the next. No treatment available.

Acers tend to suffer from dieback anyway, so its more likely the problem with your Acer is unrelated to the problem with the Berberis, if they are far apart. If its in intense sunshine and heat, it may well start to frazzle, and may recover once the very hot weather has passed.

Re. the acer - would it be better to move it in the autumn to a more shady area ready for next year?