I'm going on a trek to south western china and cannot find any good maps
of the Khawa Karpo area

try this.  hope it helps you in your search
http://www.mindbird.com/china_taiwan_tibet_maps.htm#tibet

I doubt that you are going to find any maps useful for trekking. 
I suggest getting "Tibet Handbook" by Victor Chan, Moon Publications,
and now I find a very promising book - "Mapping the Tibetan World" on 
www.tibet-travel.com.
The handbook only has sketchy maps with descriptions of various
pilgrimage routes, but the book is a wealth of 1000 pages of info on
Tibet and its Buddhismus.
Good luck.

greetings again,
check out "tibetmap.com".  It will give you some overview of what maps
are available.