Radiator bleed valve stuck. Nipple rounded off. Problem solved. Inexpensive and easy fix.
Brake Bleeder Screw Removal Tool - 1A Auto
Unable to loosen bleed screw for radiator in desperate need of bleeding, have tried all sorts of tools, would removing the hexagon nut to replace it cause a leak? : r/DIYUK
Radiator bleed valve stuck : r/DIYUK
Rad bleed valve broken? A last resort tip. A follow up video on how to renew a bleed valve.
Bleeding Central Heating Radiators | Problems Explained
Radiator-parts: Removing Bleed Screws
Stripped Bleeder Valve | Chevrolet Cruze Forums
uk - What is this kind of radiator bleed valve called? - Home Improvement Stack Exchange