Owner of the flat must be in breach of their lease so normally, contacting/informing owners/managing agents of the block would be a way forward, or taking independent legal action against them over 'quiet enjoyment', etc., particularly if all other residents act together. If threatened with forfeiture, the flat owner might at least get rid of troublesome residents. Danger is that in a small block they might know who complained and there'd be trouble, and it's not a quick process. If neighbours are actually threatening, a court order can help, as repeat behaviour is contempt of court and can have greater penalties than the actual behaviour. Otherwise, it's up to council anti-social behaviour enforcement team - or get the boys round!
Peter Evans ● 1917d