The whole of the pub's premises need to be itemised on their Premises Licence to be covered for drinking alcohol. It maybe that this garden is not part of the licensable premises, or if it is, then it can be revoked.
Write to the Licensing Dept at the local authority and ask if the garden is licensed. If it is ask for a licence review on the grounds that the activity is causing a public nuisance(this is the correct and relevant term) do you have grandchildren? If so mention them playing in the garden as the Protection of children from harm is another grounds for a licence review.
The result could be closure of the garden, building a wall, or reduced hours. You might like to think what solution you'd accept but ask for closure of the garden from the start and see where you end up.