I have similar problem to another member - rad hot at top, coldish at bottom. Bleeding is the first thing to do. Broke two keys trying to free the bleed screws. What do I do now?
By the way the square heads are on the inner face of the double rad so even if I remove the rad I still cannot get to the head of the screw, which is almost 'buried' in a kind of pocket (why I do not know). A larger tool would simply be obstructed by the pocket diameter which is only just big enough to accept a standard bleed key anyway. You couldn't even get in there to drill out the bleed screw- unless you had a flexible drill bit........???
As you might have guessed these rads are not exactly "top of the range"...:=)