Hi Napalm I'm a plumber/heating engineer
You could close the working radiator valve and remove the rad, then if you have the right fittings you could make up a temp drain point using the working rad valve as the drain and running a hosepipe from that. As you are probably well aware your system will be fitted to a high pressure so be careful. It's not a hard job to put it right as long as you have the right fittings, tools and understanding. If you don't have all those then you're better off calling a plumber
Good luck