Here is an old mechanics trick............get a couple of detergent tablets that you use in your household dishwasher.Break up one tablet and put it in the radiator,go for a 10 min drive with the heater turned on full, come home let it cool & remove the thermostat and flush it out.Refit the thermostat and do the whole process again.I have been using this method for years and it works a treat.Just make sure you use tablets for a dishwasher as they do not foam up.If you use liquid dishwashing detergent that you would use for washing up by hand it will foam up like it has rabies and overheat.Good Luck
