I have a 1999 XK8 that has the metal thermostat housing and Iv'e been chasing a faint coolant smell and I found a small leak on the thermostat housing and I need to order the seal that goes to piece that houses the thermostat and has the three bolt pattern. one source shows a thermostat with the seal attached one source shows a separate rubber seal.

 

Get the one with the gasket, get new all around. Pay attention to the hole in the thermostat and make sure in the right position upon install.
Goes at the top if I remember right. Draining the system too below that area is not to bad, just suck it out with a turkey baster after taking the top off the housing, might want to get that gasket too.

 

The thermostat seal is sold as a separate part. Attached is the results for a parts search from SNG Barratt ( a forum sponsor).
The thermostat is designed to be installed without a gasket in the housing. Using a gasket defeats the seal and put the system pressure on the paper gasket. Chance of failure is greater with the gasket. Just make sure the thermostat cover is bolted down squarely to the tower.