Is the cooling fan kicking in? If no, remove the fan thermostat switch from the radiator and bridge the two connection. If the fan runs, then you need a new fan thermostat switch, (about £10 -£13 from eurocarparts.com)
If the radiator is cool and the rest of the engine is burning hot, then there could be a couple of things.
1, The engine thermostat is buggered.
Test this by removing it and suspend it in a pan of water,(if should be closed) bring the pan up to the boil and it should open, if it is not fully open, you need a new one.
2. Make sure that there is not an airlock in the system. This just the same as in your home. Open the bleed valve on your radiator and run the engine, any air should be expelled. Run it till fuild just starts to come out of the bleed ole and tighten the screw up again and top up the coolant as necessary.
Phill
June 2009