For anyone interested I found the info I needed.
PP6 is the ground
PP25 is left audio channel
PP26 is the right audio channel
I soldered wires on these points on the underside of the pi and connected them to the amp:-
PP6 to L- and R-
PP25 to R+
PP26 to L+
Connected power to the amp and voila, I have sound!
Now i just need volume control. :-)