Hello again.. I have now tried a new 2A PSU (plugged directly into the PI and bypassing my lights) and have exactly the same problem.
Infact I have tried a few PSUs, all good quality (Apple, Samsung) and am not seeing any improvement.
I even went back to my old SD which has OpenElect (built on Noobs) and thats the same, so I know its not related to LibreElec... Do you have any other suggestions?
Jon
Edit: i did consider a fault on the Pi3, but i have connected a USB keyboard and 12 hours later, its still working fine,