(and are these really errors or is Reimage inventing them to justify the cost)?
You have probably answered your own question :0)
would it be better to leave windows running overnight instead of switching everything off when I go to bed, as I assume any errors are a result of loading and unloading windows?
I always switch off when not using the PC / laptop (why waste electic an a laptop is switched off when being carried from one location to another.)
As long as windows is closed down properly and you o not just yank the cable out then it will not get any errors just booting up and down normally.