Ok, possibly another piece of the puzzle.
I can't replicate the issue at the moment as I now seem to be having issues with the HDMI CEC, It was set to put my devices into standby, which it did (amp and TV) when the screensaver activated.
The resolution issue occurred when the TV and amp came back on, but now neither are switching off when the screensaver activates.
Settings are as they should be in the CEC section of Kodi. Will tinker a bit and see if I can get the amp and TV to go into standby as they were doing this morning.