Thanks Saurabh, but I've already tried various newer images and can't get any to boot at all (OEM logo shows, nothing else). Not sure if I've tried the very latest builds, so will do that.
This box is based on the p281 board (according to the Android "about" page) and is branded as MXQPro 4k. I have tried with no dtb, gxl_p212_1g.dtb and gxl_p281_1g.dtb - the last of those presumably the correct file for this board. The only image that I've been able to boot is Armbian_5.37_S9xxx_Ubuntu_xenial_3.14.29_mate_20171226.img. With that image it seems to make no difference which dtb I use, or none - works, but no wifi, remote doesn't work , and only 1 of the 4 USB ports work. The one USB which works is at the back of the box next to the SPDIF socket, the other 3 are on the side of the box with the SD ram socket.
Going to try again now with Armbian_5.41_S9xxx_Ubuntu_xenial_4.9.40_server_20180315.img