radiator bleed valve problem

have just got round to turning heating on and bleeding radiators - alas cant turn the bleed key in one of them - think am in danger of rounding it off - have tried using pliers on it - any better method - or do I need more brute force?

also on the same radiator - it's a double the other half of it is installed so close to the wall can't get the key behind it - so have always had one half cold

Usual disclaimer - I'm not a plumber.

I've had a similar problem on one of our old rads - I got some pliers and unscrewed the screw from inside the bleed plug completely, and replaced the screw with a new one from a brass air vent fitting from B&Q. The system was filled, so there was a bit of water on the floor in the end, but it worked fine.

Pliers will only make it worse, as they;ll round off the head.

You could get a new insert plug complete with drain screw.

Might as well do both sides while you're at it.