My recent purchase

[Rubbond]

If you still have the old thermostat, wife permitting, you can easily check
if it is/was working by putting it in a pan of water and bringing it to
boil.
At least you will know if a faulty thermostat was a contributing factor.
I would at least take off the top and bottom radiator hoses, and flush both
radiator and the engine seperately. See what comes out.
Good luck!
Andrew.