Update.
These pens work perfectly with the i86; they're cheap, and the tip is a soft rubber, which won't scratch the screen.
(
http://www.amazon.co.uk/gp/product/B...ilpage_o01_s00)
Regarding the device, I am sending it for replacement, and I imagine I must have a defective model. It never willingly – and now does not at all (except via xodo, a foreign app) read the SD card– and it is not comprehensible how the battery life should be so poor, especially given that it performed much better originally.