I would do what QinteQ said and check to make sure the cooling system has been bled properly. Potentially the new stat could be stuck in the closed position, unlikely but possible. I have had a brand new one luckily stuck in the open position. After that then you will be looking at potential head gasket failure.
With reference to doing the job yourself I 100% agree, however I also know that there are a lot of people that lack the knowledge and/or confidence to do the work themselves. I would always offer out my services to help either online or at my home if you were in and around Dorset, UK.

 

If the engine has been run seriously with an air-lock in the cooling system, it's likely the head gasket has ruptured giving the symptoms described. This is even more likely with the high compression and firing pressures of a diesel.
In the 1970s, Hillman Imps were (in)famous for air-locks in their cooling systems due to the long coolant lines from rear engine to front heater core. Ruptured gaskets and distorted cylinder heads were rife with these. The situation made even worse by corroded steel studs on the aluminium block. HOWEVER, if the job was done properly using the bleeder taps provided by the makers, and following the instructions again provided by the makers -- no problem. Also, if the anti-corrosive anti-freeze was changed as per the maker's schedule, again -- no problem.
Another example of unnecessary problems caused by incompetence.

Leedsman.