The cooling system bleeder screw is located at the highest point in the cooling system on one of the heater hoses. It allows trapped air to be bled from the cooling system. The screw is made of plastic and is prone to breakage.
Some might consider this a permanent repair. Obtain any commonly available heater hose with a 90 degree bend. One such hose is the Gates 19604, which actually has two bends. Reuse the spring-type hose clamps, cut off any unneeded length of hose, and insert it as a replacement.
If your bleeder screw gets broken, don't give up on the hose. You can use traditional screw extractors on the remains of the scew. You'll be fortunate if the screw extractor succeeds in extracting the screw, but at the very least, a reverse-threaded type screw extractor should succeed in removing some of the plastic material. After removing as much material as possible this way, use a 6mmX1.0 screw tap to remove any remaining plastic material. This is best done after removing the curved section of hose from the vehicle so that particles aren't allowed to get into the system.
To replace the original screw with an identical plastic version, it's necessary to replace the hose. The hose can't be purchased separately, and can only be obtained as a component of a larger assembly that contains two T fittings (one for a sensor and one for a hose) and several sections of hose.
Another alternative is to purchase a replacement brass screw, which is currently available from eurocampers.com and also, gowesty. Note in those photos that a slot is cut into the threads. This allows air to pass without the screw having to be completely removed.