My concern is really whether I'll have sufficient control via using the 100mms: with the darkest section at the very top, I'm not sure I'll be able to use it without having the whole scene darkened on my smaller-element lenses.

Sorry I'm not worried about the 100mm fitting the Sigma (the grad maker I'm looking at, Formatt/Hitech, has a special wide angle holder) - I'm worried about whether on my Pentax lenses (which do not have the Sigma's huge front element) the filters will be unsuitable because most of the gradation is spread over an area the lens doesn't 'see'

I was looking at getting a set of Hitech's 100mm ND grad filters for my Sigma 10-20. Will these be suitable for use on my Pentax lenses, or will the graduation be spread over too wide an area? I'm loathe to get two sets right at the start, so if I can't use the larger filters on my other lenses I might well get the smaller size ones (my kit lens is perfectly wide enough for sunsets anyway) then a larger set for my Sigma if I end up getting great use out of them.

The Pentax is better

That baseless fanboy remark aside (I have never used either lens, nor even read other people's opinions of the FD), Canon FD lenses will only mount on 4/3. Whereas K lenses will mount on Canon's APS-C DSLRs (not on FF for mirror reasons), Sony Alpha, 4/3, and of course Pentax DSLRs whose owners are fleeing to old manual lenses after last year's price hike.

It might seem relatively low as a percentage, but it is in my opinion still high relative to the norm for such things. We do not read the same noticeable numbers of Pentax body users having the HSM on their Sigma lenses fail. (front-focusing is another matter! I mean HSM working properly when the lens arrives but then failing - the problem for which SDM is under the spotlight)

Surely with Sigma's reputation for poor quality control, and the fact that (with current Pentax prices) many Pentax body owners are buying Sigma HSM lenses, there should be an equal if not greater volume of complaints from Pentax body users about Sigma HSM if in fact SDM does not have worse problems than other brands.

The Pentax 17-70 has a very good reputation, the only things stopping me from buying one are the lack of weather sealing and the SDM (though the DA*50-135 is likely to severely test my SDM aversion at some point). As alternatives you could consider:

Tamron 17-50 (still f2.8 and reportedly lightish for an f2.8 standard zoom)

Sigma 17-70 OS
*About £120 cheaper than the Pentax
*Faster wide open (f2., and no slower at any point in the range (slowest is f4)
*Uses Sigma's HSM, rather than Pentax SDM. This is generally reliable: so long as the lens isn't front/back-focusing when it arrives (in which case most reputable vendors pay return postage), there are very few reports of Sigma HSM failing.
*Has Sigma's in-lens OS. This is reportedly quite good, and if you don't like it you can always turn it off and use the camera SR (NB: Don't use both at once).

A first useful stop would be a program called ExposurePlot. It will analyse all of your photos (JPEG format only - if you use raw then ImageReporter [Lightroom only] is what I use) and give you a graph of what focal lengths you used. This would tell you whether a 17-70 will be any more useful to you than an 18-55 (e.g. if most of your shots are only above 55mm, whereas if lots are over 100mm then it wouldn't be).

As far as superzooms go, the Pentax 18-250mm is the best (or the least bad), and the Tamron 18-250mm is actually the same lens (perhaps excluding coating) so if you are buying a superzoom lens buy whichever of those two you can find for the cheapest. I'd still echo the above though - a superzoom will not produce images as good as your kit lenses.

Helios 77M-4 - reportedly has more resolution than the 44 series, and is 50mm f1.8 rather than 58mm f2. Lovely little lens - I suspect that my very, very poor MF skills are the limiting step here rather than its quality.

One note is that it doesn't have an auto/manual switch. The fix that had been done by the bloke I bought it from (superglue over the aperture pin to hold it down) gave way soon afterwards. A small piece of an AOL CD from a supermarket (they really are multi-purpose!), edges filed smooth and loctited into place, makes for a far more permanent fix

Wrong, unlike the Harlequin Kx which was adorable genius.

I suspect the article was somewhat tongue-in-cheek, the author having come to the same conclusions about this... event... as we have!

As far as flashguns are concerned, the Metz 48 AF1 is cheaper and more powerful than the Pentax 360. Its only flaw is its Byzantine menu system (I have yet to get it to fire in wireless mode, having owned it for six months )