Officially it's Ctrl+F11 & Ctrl+F12 or KEYPAD
7 & KEYPAD
In practise it's a bit quirky.
Specifically it's Left Ctrl+F11 and Left Ctrl+F12 to
switch to previous orientation and next orientation respectively.
You have to release Ctrl before
you can rotate again.
KEYPAD 7 and KEYPAD
9 only work with Num
Lock OFF (so
they're acting as Home & PageUp rather
than 7 & 9).
The only orientations are vertically upright and rotated one quarter-turn
Maybe a bit too much info for such a simple question, but it drove me half-mad
finding this out.