Seem to remember the bung and seal was around £5-10 from Ford. No you don't need to remove the housing, you might loose a litre of coolant, but catch it in a clean tray if you can then pour it back in. Its easy enough to do, you just can't really see what you are doing as it is underneath.

This is the part, on the RS the hole in the housing is used for a water feed to the turbo, but the ST170 doesn't need it so it has a bung fitted.