Update: Over the last few days, I've found UK listings for the PL15 at exactly the same price as the S16, but even at the same point, I still gravitate towards the S16 for my requirements. The pros for the PL are its focal length, and in this range, 1mm makes a difference, however a prime is always a compromise, sometimes it's too wide, sometimes too long. The other big pro is it's colour rendering, but colours can always be manipulated in PP.
Pros for the Sigma are its extra 1/2 stop of light gathering, and it's the sharpest of the bunch, these are both key advantages for my intended usage. The lower the iso and the less sharpening I need to apply, the better. The trade-off is its size, but I see no real world difference to using the 12-40 or 45 f1.2, they're all in the same ballpark. Discretion is not a concern, I don't do street, and I rarely photograph people without consent. There's enough difference between 16mm and 20mm to justify a benefit to me, and I've the perfect size space in my camera bag for it. it has the best bokeh of the bunch. Throw wr / dust sealing into the mix and it simply ticks the highest number of boxes.
To my eyes, the Oly 12 f2 and 17 f1.8 fall slightly behind in terms of IQ compared to the other 2. The 12 is possibly too wide, I don't need this for landscapes and quite often I'll be shooting a subject from behind a barrier. The 17 is obviously the closest to my P20mm, so it has the least benefit.
Yesterday (as I expected), Sigma launched a November price promotion, which has dropped £75 off the 16mm, and it seems rude not to take advantage, so I've placed an order.
Thanks for all your input, whilst I didn't follow the popular consensus, it was a great help.