I'd like to minimalise clutter by clearing the Post Icon and Folder icon columns from the main thread display - but when I clear it from the ForumBits template, the table goes all mental and columns get all mistakenly rendered....

is there a clean way of doing this?

when you take away the folder and post icon columns you have to edit one more template. Open "forumdisplay_threadslist" and search for this:
<td bgcolor="{tableheadbgcolor}" align="center" nowrap colspan="3">

and replace it with:
<td bgcolor="{tableheadbgcolor}" align="center" nowrap>