There is also a suggestion, apparently based on Photozone reviews, that at the same focal lengths and apertures the Sigma 10-20 actually edges the DA15 for IQ until you get to f8 or so.

The 'problem' with the Tamron/Pentax 18-250 (the 'Pentax' lens was a rebadged Tamron) is that it is : a) Discontinued and b) Popular. That means that there aren't actually all that many available second hand.

In terms of image quality, you would be better to buy the 55-300 others have mentioned (and take your 18-55mm kit lens too, for wider shots) - this will also be substantially cheaper than the 18-250 (which was £400-odd new IIRC). However the 18-250 is widely regarded as the best of the various 'superzoom' lenses (ie those which go from 18mm to 200 or 250mm).

Sure - much of Japan's eastern coast either flattened or irradiated, and protests against the biggest cuts to public services in Britain's postwar history. Saving 1/1000000000000 of the carbon dioxide of the coal power plants China built this week has much more long-term significance

Royal Mail charge £8 on top of the import fees, or at least they did last time I bought something from the US. As regards how much you will be charged by Customs, this site is your friend:

http://www.dutycalculator.com/

As a proud owner of the 35 Limited, I can vouch for its image quality in non-macro situations. It's great wide open (I actually like it for small-group portraits), and stopped down to f5.6 or more the sharpness is just silly. Of course like all macro lenses the long focusing range can make it ponderous if the AF misses focus at the first attempt, but it is the least prone to this of the three macro primes I own.

On the subject of the 50-150 being discontinued:

http://www.sigma-photo.co.jp/english/news/110208_apo50-150_28ex_dc_os_hsm.htm

Sigma's lens OS is apparently quite good, but it does seem to have significantly increased the heft of the 50-150, whereas petiteness (as well as price) was one of the old one's big advantages over the 50-135.

I had two Sigma 50-150s and they both focused blazingly fast on my K20D. Unfortunately both copies front-focused like mad at 100mm and above (not the camera, because I got out the DFA100 for comparison and it focused fine for the same shots/light), so after the second one I gave up and got a DA35 Limited instead

The weird moustache-distortion only happens at 10mm. Anything upwards of that and it's fine.

Rarely, though, do other brands allow their flagship professional-grade zooms to become a running joke to such an extent that people buy Sigma zooms for reliability (great as 'modern Sigma' QC is, try finding a Canon user who favours their reliability over that of the L series). If the DA Limiteds represent the best of Pentax's modern lens design (not a verdict I'd take issue with), then the ongoing PR fiasco that is SDM must surely represent the worst

EDIT: On the topic of the 16-50 specifically, aside from SDM issues they are reported to have a higher-than-usual incidence of 'decentering issues' (ie being built squint). In fact this has been suggested as a possible reason why the 16-50 and 50-135 seem more vulnerable to SDM failure than other lenses with more glass to move (e.g. the 60-250) which use the same motors.

It has never showed focusing problems, but (according to your signature) you don't have any other f2.8 AF lenses. So maybe you didn't notice before because of the greater 'leeway' for focus accuracy at smaller apertures?