Changing Thermostat Housing

Hi All

Been a while since I last posted, typically when I do its when the cars gone wrong :(

I changed about two months ago the engine coolant sensor and temporarily this sorted out the temp gauge not moving and the engine management light coming on. Last night towards the end of my journey noticed the sensor hadn't moved baring in mind I had done roughly 20miles and the engine management light came on again. I'm thinking that I need to replace the thermostat housing now, am I correct in thinking that?

If it is the housing how much work is involved in replacing it? Also being a bit dim this morning where is it on the 1.4 8v engine preferably pictorial help?