You need to make a complaint to the environmental health Department at the local council and also to the housing department.
Whilst there is nothing actually illegal in them doing what they are doing provided it is an open space, what they are doing is causing nuisance.
That is a breach of the terms of their tenancy agreement with the council.
If the local authority will not do anything about it, then you have no option but to take the neighbours to court for an injunction to stop them doing this on the basis that is private nuisance which is affecting you.
Before rushing off to court, and assuming that you had a word with the neighbour, and the local authority are unwilling to help, a solicitors letter may assist to get this stopped for incurring potential legal costs.
There is no legal aid for this.
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