K11 Overheating

My trusty K11 (2002) has started overheating, we thought it might just be the summer, but it got really bad today and nearly got to the red (about half way), the radiatior was replaced on it around 6 years ago and has been fine since.

Some background:
Putting the heater on cools it down.
Water appears to be coming out the overflow tank.
The radiator is warm all over (no cold spots).
Both entry and exit pipes from the radiator get hot.
There is no water in the oil or oil in the water and no visible leaks around the engine header etc.

I'm thinking it's either, the thermostat, the radiator or the water pump.
Is there an easy way to check all three?

Thanks in advance.
 
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