Defender 300tdi mystery overheating

Whilst driving home today my temp gauge went off the scale, so I stopped. The heater was blowing very hot. I removed the coolant reservoir cap and to my suprise there was no sign of any pressure in there, and the coolant in there was cool. I checked all the hoses and all of them were hot. Once it cooled down I drove away and it overheated again almost immediately.
Any ideas of the cause of this? Where do I start?
Thanks in advance
Rich

radiator hot all over ,water pump impeller still turning,expansion cap holding pressure

Thanks jamesmartin!
Looks as though the rad is shot. The inlet/oulet side is getting hot but the core and the other side are stone cold. I just pray that the head will have saved.
Any suggestions on where to buy a decent rad?
Cheers
Rich

ebay,btp2275

Many thanks again!

budget radiators, rotherham. advertised on fleabay too.