Just had a go in a hire car that a mate has. It is a petrol Fiesta that has a TDCi badge on the back!
The engine is a little rough at the top of the rev range but feels fairly nippy. I'm guessing it's a 1.4? The engine bay is reasonably full, but still quite a bit of room left. It says Duratec Ford on the engine cover.
Had a good blast with the handling and was pretty impressed. Good resistence to understeer, excellent body control, very controllable on the limit. Fun. A million miles ahead of most of the competition - i.e. Corsa.
Interior feels pretty cheap which is a shame.
The standard Fiesta is damn good, so the ST and RS should be amazing!