It might be a marginal gain to remove the effects from smoking but it's still a gain. Why does it have to be a case of "you must tackle the biggest issue or not to anything at all"? I'd probably agree that it's not going to make a huge deal of difference to air pollution on anything but a very local level but then again tackling small problems can be a good way to lead up to tackling a bigger problem. If nothing else you remove the opportunity to get distracted into other areas as it will only leave the bigger issues.
I've got to say that I don't think the director of FOREST is really helping his cause by stating that there are no health risks to anyone apart from the smoker - there's been evidence for a number of years that passive smoking can lead to problems. That's not to say that you can't find studies which debate the level of the impact but to baldly state that it's not a health risk for anyone is leaving yourself open to challenge.
I'm also curious as to why people who don't like the smell should be the ones to "walk away" - smoke doesn't tend to respect boundaries and will waft from place to place. How far does Mr Clark propose people go and why does he propose to inconvenience people who aren't the ones causing the issue in that area? For what it matters if people want to smoke that is their choice but there also needs to be an awareness on all sides that their choices and demands/requests may lead to inconveniencing other people - effectively a variation on "my right to swing my fist ends at the other fellow's nose".
the issue i have with this is that its part of a wider report to supposidly help londoners get healthier yet skirts around things, as iv said a page or two back, the report has 4 pages of anti smoking info and 1 and a half pages of info about vehicle fumes, even though the latter probably does more damage over all. also the fact that cycling only gets a single mention in a bullet point.
just seems a bit cack handed for them to go after the smokers once more because its easy rather than actually go after things that would make a real difference. but yeah a ban would be an improvement but in london with the amount of traffic fumes will you actually notice it ?

(sorry couldn't resist!)