Most people with these kind of problems have been suffering prolonged drought after a severe winter - not sure if that's the case with you or not, but that would account for slow growth - unless its a case of Viburnum beetle infestation. Feed and water if you can - I know even Scotland's not had a lot of rain, so I'm assuming you're in the same boat.

how do you get rid of vibernham beetle on a large bush?

Spray it comprehensively with Provado Ultimate Bug Killer. Best to buy the concentrate and use it in a spray.

It is best to read about the life cycle of the beetle for the timings to spray as there is no point in doing it a the wrong time. I had to hard prune my bush in the end and then spray every bit including the trunk. I also watered provado into soil underneath and mulched with garden compost. I am having to do it again this year as there are a few still there. I do not like chemicals but the beetles decimated my shrub. I understand the beetles lay eggs late summer which overwinter, and the larvae hatch in spring. By mid-May the larvae are eating the new foliage, and in June they move down to the soil to pupate. The adults emerge in July and August. The cycle goes on!!