*NORTH AMERICAN (Check to ensure predatory insects can be introduced in the UK)
There is a pretty good research article from Cornell University. It does not specifically list pesticides that you could place around the base, however, there may be a few other options that you will find suitable.
Such as inducing a predator insect to attack the beetle:
Encourage beneficial insects. Several generalist predators feed on viburnum leaf beetle larvae including lady beetle adults and larvae, lacewing larvae and spined soldier bugs nymphs. The lady beetle adults and spined soldier bug adults also eat adult viburnum leaf beetles. Encourage these and other beneficial insects by avoiding broad-spectrum insecticides and maintaining diverse species of plants for beneficial insect habitat near viburnums. Initial investigations by Cornell researchers on these biological control options also indicate that augmenting natural populations of these predators may dramatically limit viburnum leaf beetle damage. Researcher continues in this area, check back later for more information.
Prevent the larvae from maturing:
Foil larvae. Researchers recently learned that the larvae don't drop from the leaves to the soil to pupate. Instead, they crawl down the tree. While it hasn't yet been tested, it is likely that putting a sticky barrier such as Tanglefoot tree pest barrier around the base of the shrub could keep larvae from reaching the soil to pupate.
Given your situation the other options listed on the site may not be easy to accomplish. These solutions may take time, but in the long run should protect your tree.
(Information directly from link posted above)