larknz: It could also be a loose connection in the plug
Trying a different extension cord and wall socket, that should work it out. It could just be that running 9A through a cord for a solid hour creates heat.
Watts total is not what matters in a case like this, unless you coil it all up in a pile (you should not do this unless it's a very small load - it cuts the capacity by a lot). Watts per meter is far more important as more meters means it can shed more heat.
To get 3V drop from a 9A load on 1mm^2, GenCalc says you would need 7.2m, and it would be running at about 44C if in 30C ambient and laying against a surface but otherwise unenclosed and with no other cables in close proximity.
The maths and experimentation has long been done on this. ASNZS 3008 is 150 pages just on sizing cable.
If it is hot the whole length of the cord, then that's the cord itself getting warm from load. Hot only at the ends (or one end) indicates a contact or termination problem.
I changed my office fan heater to a new extension cable in a new socket. The cable still doesn't get warm, and the end plugged into the wall doesn't get near as hot. I need to mix and match a bit to work out if it's the extension cord or the wall socket, but with a 1000W oil heater using the cord / socket that was getting hot before it's now cold, so probably at least worked around if not solved.
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