Thanks for all the advice above.
I’ve gone with a mullet build in the end, 27.5 (2.6) on the back and 29 (2.35) on the front with a 120mm 29 fork, seems to be pretty well balanced and doesn’t look obvious. Build has come in at 13kg, Size M fits my 170cm 12yr old perfectly with hopefully a bit of space to grow, she’s well happy with it.
My pal who’s a very good rider and the right size (I’m a bit big at 6’3″) had a razz on it yesterday and was vey impressed and didn’t think the geometry was compromised as a mullet for climbing.
edit: bah, cant get image to work but hears the direct link:
https://www.flickr.com/photos/191420910@N06/shares/A47M80