Rounded off bleeding nut on radiator

As the title says, the nut has been rounded off a tad and the last time I bled the radiators (ie. when we moved into our house) it finished it off What's the best solution ? How do I get it our so i can replace it with a nice new one ?

Thanks in advance.

First, does the female bit unscrew so you can replace the whole vent, like one of these or maybe a flatter version:



If not, file your closest-fitting rad vent key as though it had been in a pencil sharpener, so you get the best chance of getting to the bottom of the square. Often they're concave ended.

You can use a dremel 20mm disc to cut a slot then use a screwdriver.

I have some angled mini locking "Vise-Grips" which often work.

Also sockets from a 1/4 drive socket set but they must be the 12 point type, which are rare. So every 3rd angle makes a square. If you have imperial and metric, one of them is usually a good fit.

Fantastic set of solutions Chris ! Many thanks Have a great weekend.