Toyota Lucida 2.2 Diesel, thermostat stuck. Help please!?

I've been trying to replace the thermostat in my Lucida 2.2 diesel auto. I've removed the stat housing, but the thermostat is impossible to remove. The top part has broken away from the lower section which remains in the housing as though it is welded in place. I've spent an entire day hitting it, pulling it, bending the perimeter inwards away from the housing, heating it, you name it I think I've tried it! It is now somewhat mashed up in the housing, the sides seem to be able to flex a bit but the bottom section of the stat will not budge. Any ideas other than an exchange engine would be very welcome!

Grim.
December 2010