Untidy or overgrown front gardens are of no  concern to the council, provided they do not affect the road or pavement or that vermin are not present.
The recommended approach is to speak to the homeowner and hope that a polite approach can remedy the situation.
In the event that vermin or rodents are present, the council’s environmental department should be contacted. They will inspect and report to the homeowner as to what action should be taken it would be wise to contact the homeowner before the council is approached.
A rat or mice infestation should  also be reported to the council – main switchboard no. 020 8726 6000.