How long is someone allowed to have an engine running while they work on it?
The situation you describe is something that can be dealt with by the local authority if it is persistent and unreasonable i.e. if it is happening regularly. There are no specific rules relating to duration of engine noise of this type – just the general anti-social behaviour and statutory nuisance legislation. There is a parallel subsection relating to vehicles on the street. The fixed penalty procedure for idling vehicles generally doesn’t apply where repairs are taking place. Alternatively you could speak to your neighbours first.