Just to say - followed the instructions on the Raspberry Pi site here : https://www.raspberrypi.org/documentation/hardware/computemodule/cm-emmc-flashing.md to install the drivers and PiBoot on a PC
Plugged in my Slice using the supplied Y-Cord (the USB-A connector you plug in to your Computer is the one with the USB logo on, which has cables to both the second, power-only USB-A, and the Micro USB-B) and then plugged in the power supply to the Slice.
WIndows detected the internal HD in the Slice (I have a 1TB model) before I supplied power, when I supplied power the HD unmounted, and Windows started installing drivers for the CM etc.
Once the drivers had installed I ran RpiBoot on my Windows PC, which mounted the Compute Module Flash as a drive (D in my case), then used the LibreElec installer available here https://libreelec.tv/downloads/ to flash the img.gz downloaded from the link above (ignore step 1), selecting the .img.gz as the source image, and checking that the right destination drive had been selected.
All ran like clockwork, and the Slice is now happily running with LibreElec Beta 7.95.1 on a CM3.
I'm unlikely to run HEVC/H265 stuff on my Slice, but put a small copper heatsink on the CPU (no contact to the case unfortunately - so the internals might get a bit warm as the hot air dissipates. Most of my viewing is MPEG2 and H264 576i, 1080i and 1080p - so the CPU shouldn't get too stressed.