You dont need PTFE!!!!
There's an o-ring in that valve and now you've desturbed it its leaking, turning both valves off should stop the leak.
You can also try turning the valve on and off a few times, bed the o-ring back in or shift any crud thats making it leak, may work to stop most of the leak. Or open it all the way works sometimes too, bit of a knack to these things that only comes with experience!
You could very well get some freeze spray and change the valve yourself, you dont need to drain and remove the rad though. If the ice plug melts though you're screwed. We have an electric pipe freezer as the cans just aren't reliable enough.
You can also get plugs that you plug the cold feed and expansion with, this holds the water in a vacuum so swapping a rad valve is easily done. This is assuming you have an open vented system, if you have a pressurised system its even easier as you just have to let the pressure off.
Now as long as you manage to contain the water you'll get while changing the valve you're good, things can easily go wrong though and there's a lot of water in that pipe and it'll cost more than a plumber to repair the damage it causes if it leaks.
Basically its easier said than done and its very easy to get out your depth very quickly.
Oh, and if any of your rads suddenly stop working its not too likely to be an air lock and you wont remove the air by venting the rads anyway, it'll end up in a rad after you've shifted it from the place it was locked.
