Should I add a diesel additive to keep my DPF working properly?

Is it worth using a DPF additive in a tank of superdiesel to keep my car's DPF clean?

Is it worth using premium grade petrol?

Is premium grade petrol good value?

What Super fuel do you recommend?

I've followed your advice for a number of years regarding premium fuel, both petrol and diesel. I usually put BP Ultimate or Shell V Power in both cars, a petrol MINI and a diesel BMW X3. I appreciate...