hot water / radiotors too hot

[JB07]

Help?
we have just had a second relatively new radiator (less than 4 years old) develop a hole, the first one to go was the one in the hallway nearest the room thermostat, since we turned this off it has been noticeable that both the hot tap water and radiators have become too hot to touch culminating in the second radiator (nearest to the one that had already developed a hole) developing a hole at the top of the radiator and thus creating a blister type bubble full of water at the top of the radiator.
can anyone please help diagnose this problem, why the radiators are getting getting too hot and then developing leaks and the water being too hot too touch?
Don't know whether this is connected or not but the small expansion tank in the loft is over flowing (the washer needs replacing) this tank contains hot water? Someone told me that the expansion tank should NOT have hot water in it?

 

[TicklyT]

The feed and expansion tank should not get hot.

One cause of hot water in the tank could be 'pumping over', where the heating system pumps water out of the vent pipe over the tank as it runs. This aerates the water in the system. Normally, there should be no water escaping from the vent.

Air in the system leads to corrosion and premature failure of your radiators.
Sludge builds up, causing more pumping over
etc.
etc.

The initial cause of pumping over include poor system design, a system in desperate need of cleaning and maintenance, or a faulty thermostat in the boiler