Well part 'a' applies here, i.e. the condition of the vehicle causing danger of injury. Exhaust flames fall into this defination as they could indeed harm someone crossing behing, a biker close behind, etc.

You may not like or agree with it, and perhaps you can't see how your exhaust flame may be a danger, but this section alone of the Road Traffic Act will be enough more than likely enough to report you for the offence.