Maybe this site can help:

8 natural & homemade insecticides: Save your garden without killing the Earth
https://www.treehugger.com/lawn-garden/8-natural-homemade-insecticides-save-your-garden-without-killing-earth.html

Spray with Ultimate Bug Killer and pick up any debris under the bush. Unfortunately this will be an on-going problem and you would need to spray every year (April) as a matter of course. The Tinus varieties are probably the most susceptible. Even though you can eradicate them from your shrub they can re-infect from neighbouring plants.

I had that on my Viburnum last year. I cut it right back and sprayed what was left with bug killer. I lost one season's flowers but it is now a beautiful shrub again.

Not much point in spraying with insecticides now,, the adult beetles are much less susceptible and you won't get rid of them by spraying. Much better to put up with them in the meantime and wait till next spring, around April, when the larvae have hatched and are nibbling on the leaves, then spray thoroughly, including the backs of leaves, till run off. Suitable insecticides mentioned in the link below, but I can tell you that Bug Clear Ultra does the job with one spraying if done properly at the correct time.

https://www.rhs.org.uk/advice/profile?pid=556

I have struggled for years with the same problem, last year I bought nematodes from an online supplier and instead of spraying I gave my shrubs a good douching in autumn and spring, made sure the ground around said shrubs had a good soaking, I have new heathy growth on one shrub and for the first time in many years the other one had many flowers, tried sprays in the past without success, I also keep dousing the shrubs with homemade Garlic mix, only time will tell if I will have succeeded...

Thanks everyone for your advice and suggestions. Think spring approach will be most effective. Trouble is I can’t reach half of hedge so difficult to spray.

If you can, cut it back fairly hard first to make spraying easier/more effective.

My experience is the same as Hywel's.