Originally Posted by fards
that sounds like you haven't got the stock recovery then (recovery is volume) (fastboot is camera)
Well that's the bizarre thing: I didn't think I'd changed the recovery, having only flashed stock - 4399 - 6941 (I have to confess my relative ignorance of the Android way of things though). I did download stock as linked on THIS
useful post by lufc, and installed it as per his instructions.
EDIT: sorry yes fastboot = camera, I used that to reflash the stock recovery. Then use vol key to boot into it for update flash.