Thanks ... Chipp is right it's an edge issue - classic AA problem!

The edges in question have a pretty plain white-ish material, no textures. Those edges are the skirting boards along the base of the wall, which under low-frequency photon condtions, reveals the weaknesses in the lightingAA interaction.

Yep - it's definitely a "straight edge + lighting" issue. Some of the other edges in the image are beautifully smooth, even those very slightly off the horizontal or vertical are silky smooth, but they are the ones in high-lit areas, whcih suggests it isn't just an AA issue.

I might need to remodel the skirting boards to project further from the wall and to have a larger chamfer and see if that helps.

Thanks
Richard