I couldn't find anything in the AMD control panel about mip-mapping, but i have most things set to high-quality or application controlled anyway.
and on a serious note, i'm redownloading from steam (taking ages) and hopefully the re-install will put things right. let you know soon.
This is almost certainly a Catalyst issue. Go to the control center, Gaming > 3D App Settings > Disable Catalyst AI, Mipmap Detail Level = Quality.
That will solve most issues with games. Here are my full settings:
SMOOTHVISION Anti-aliasing - App controlled
SMOOTHVISION Anisotropic - App controlled
Catalyst AI - Disable checked
Mipmap Detail - Quality
Wait for Vertical Refresh - Always off
Adaptive Anti-aliasing - Unchecked
OpenGL settings - Unchecked