Every person not being employed for that purpose by the commissioners, who shall make any drain into any of the sewers or drains so vested in the commissioners shall forfeit . . . F1 a sum not exceeding F2F3£25F2level 1 on the standard scale and the commissioners may cause such branch drain to be re-made as they think fit, and all the expence incurred thereby shall be paid by the person making such branch drain, and shall be recoverable by the commissioners as damages.