pannier inner bags

That's a neat find 1olbull, Thank you !

So, just to be sure (a la measure three times, cut once sort of thing) . . . there are two models listed there . . . which one please ? I presume it's the pair with the trapezoidal shape . . . just want to be sure which number is the one to order, please.

Yep - that be the one. It's called a slant.
Better call them to be sure on the size.
I use the liner bag for these "extra large slant bags".
http://www.riverroadgear.com/xl_slant.html

Just be sure you're talking about the same thing here. The R3 Touring panniers are smaller than the R3 Roadster leather aftermarket ones, they're both narrow and low. The liners I got from "TLC Products" before they vanished are rather tiny, and even with that they don't leave a lot of extra space inside the pannier when fully packed.

The construction of the Triumph panniers is awkward at best, with the intruding mounting bulges bike-side, and the bulky lock mechanism on the other side, 1olbull's "work well for me" is about the best one can do. Any bag, even a purpose-made one is going to be a compromise.
I'm also looking at the Kuryakyn equivalent bags, only because they measure a little longer -- closer to the actual Triumph pannier length -- than the River Road version . . . that approach though is the best for me.