Honestly Craig, ...easiest thing to do now you've got coolant in it again is to check whether the fans work as they should. If not then it's them.
I've only been on here about a year but have seen this so many times now, ...Jumping350, AttakZ, Ricky, Will etc have all been on here for like 4-5+ years so I'd listen to what they're saying.
It could be the thermostat but it's much easier to check the fans first, fix if needed, then work from there.