First thing to do would be change the thermostat, if the bottom hose is cold it means it\'s not opening, Unfortunately the only ones that will work are Genuine ones, Stupidly expensive here but i import mine from the middle east for bugger all (see link below, I usually buy a few things to keep shipping cost down)
https://partsouq.com/en/search/search?q=21200aa072
I have a tester but am in chch otherwise would come have a look for you.
Another option is to remove the thermostat, cut the guts out so you still have a metal ring with gasket and stick it back in, then see if it still overheats, if yes then definately HGF but if it doesn\'t replace the thermostat and take it from there.