Hi welcome to GoY.
You could spray with a systemic insecticide as they hatch and feed on the sprayed leaves they will then die.

sadly these little caterpillars are a prime food source for the smaller birds like bluetits etc, so killing them deprives food for their young. its a dilemma gardeners have to face unfortunately.

In the States, we would spray with B.t. or spinosad to reduce unintended casualties. Some of the neonicotinoid systemics can last long enough to affect birds when the berries ripen, or even kill bees when it blooms next spring. If the first two sprays aren't available in the UK, try applying compost, seaweed, and molasses around the bush, to boost its resistance and encourage earthworm activity, and plant lots of plants with small (<5mm) flowers nearby to feed parasitoid wasps, which don't sting, but do eat caterpillars.