It is an offence to cause or permit the emission of dark smoke from a chimney or flue at an industrial or trade premises. This also applies to burning materials on a site that you own or a site where you are working such as a building or demolition site or land used for commercial agriculture or horticulture.
It is not necessary for us to have witnessed the emissions of dark smoke to take action against you: evidence of the burning of materials that potentially give rise to dark smoke is sufficient. This way the law aims to stop people creating dark smoke at night and using the lack of visual evidence as a defence.
Dark smoke offences do not apply to domestic premises "except where trade or industrial waste is burnt on domestic premises". However we all have a duty of care to ensure our waste is disposed of properly.