Check the rotted stems and ground areas for pinhead size tan spheres. It may be southern blight- info on internet. I'm not sure what species are affected.

doesn't really look like symptoms of SB --- no white stuff around the rot. I did notice lots of sow bugs at the base.

If sow bugs are at the base my guess is that you planted it in very unammended red clay and the recent rains have had it rotting in the soil?

The substrate is indeed hardpan red clay, but I amended it with peat and processed manure - and it really doesn't seem unduly wet as the location is on top of a retaining wall. So who knows ?? I still think it is some bug or fungus that I imported with the purchase!

These plants like lean, dry soil and will rot if the conditions are too rich and moist. I would not use peat to amend clay soil because it holds moisture when wet and the manure provides a substantial dose of nitrogen, which the plant probably doesn't need or like. You would do better to amend the soil with a product like permatill or pea gravel, or even remove most of the clay soil and replace with a nice draining top soil.