hilary.christie, thanks for taking photos. They do however look like the original diagnosis. Flies of all kinds can be present in the garden. Some are attracted to sticky sweet residue from leaves or even neighbouring plants. Viburnum tend to be quite trouble-free shrubs, it's very unlikely to be any other pest that could strip your leaves this way.
Sounds like the shrubs are very mature, and may have been under attack from the Viburnum Beetle for a while. Action now is needed to stop the cycle. You need to be patient as it will not suddenly disappear next year once you start spraying.
The other option is to prune away 1/3 of your branches. If you look carefully, you may see signs in your branches and leaves. Look for raised round areas, or sunken round areas on leaves and branches. Removed dead looking wood too as prolonged attacks can cause some die-back.