If I floor it and go over about 3.5k RPM I see quite a lot of grey/black smoke behind me - is this normal? Besides that I don't notice anything, and have never got any funny lights on the dash when doing this or any other time.
I've started using a fuel additive (Dipetane) for the last couple of months but it doesn't seem to have made much difference (supposed to reduce smoke and increase economy) - fuel economy has actually gone down but that may be more due to the weather and traffic. Average speed last month was 34km/h (21MPH in the old money)

Should I be giving it a blast out on the motorway more often? I might go on a 120-mile trip (about 10 miles of that is motorway) about once or twice a month but besides that it's mostly stuck in crappy city traffic.