This is now resolved, thanks everyone for your help.
Despite using the stock config and reseating the CM the black screens continued.
There were actually 2 types of black screens:-
The first was caused by switching on the Slice and then powering up other devices and switching sources while the Slice was booting, this resulted in the TV displaying 'no signal'. This was resolved by creating an edid.dat file using tvservice -d.
The second is still unexplained but was resolved by doing a soft reset using the Libreelec menu. Most of my settings are now restored and the Slice is still working with no issues.