why is my 1.4 volkswagan polo overheating?

[Carl]

until recently the car was working fine, and was then involved in a minor collision, since then its been over heating,
I've changed the thermostat,
Removed all hoses and pumped water through freely
pumped water through the radiator ad it flows freely, but when i reconnect all the hoses and top up the water bottle, the top pipe going into the radiator gets very hot and the bottom pipe coming out of the rad, is still cold.
if i remove the front grill and feel the rad, only the top right corner where the hot pipe is connected has any warmth to it.
i'm completely baffled by this as i know there are no blockages throughout the system.
Any help would be great thank you.