So this is the garden bed where the ajugas failed. It was interesting to read on another blog that I am not the only person to have trouble growing the so-called bugleweed. Weed indeed, hah!
We decided to try Grevillea lanigera as a groundcover. Now that I've had a chance to go and take a photo I'm updating this post.
I would have planted the two plants an even distance from the front. The way I see it, that would make it more likely the plants will look reasonably even at the front. If a third plant turns out to be needed then an infill grevillea would look less conspicuous at the back, or there's the option of planting something else, perhaps a small strappy ornamental grass for contrast.