Had to replace the thermostat housing on my 2.5 tdi the other day, and thought I'd replace the thermostat at the same time. Anybody that's done this will know that it's a pig to hold the thermostat and seal in place whilst you put the cover on and then the bolts, and get the seal to stay in the correct place. Top tip - use a genuine thermostat as it clips into the housing, which you can then wriggle into place. I tried 3 times with a non genuine one (which doesn't clip into the housing) only to find the coolant leaking out. Hope this helps!