Ok back to the problem, I had a look on mine and the bottom pipe and thermostat inlet remain cold until the engine reaches operating temp, then as the thermostat opens the temp increases to lukewarm, letting the car idle for a while the temp increased until it was more or less the same as the inlet. If I were you I'd let the car idle until it's good and hot and check if the thermostat housing inlet is hot or cold, if it is cold the thermostat is most likely the culprit, if not and the inlet pipe remains cold with thermostat hot the radiator is probably blocked. If it were the water pump you should still get some natural circulation through the radiator and the whole system will be hot, my 2c..