What is this? Marked with an X....

And should there be what appears to be some kind of metal clip/spring just rattling around inside it? I'm guessing not. :(

yet more jobs for this lovely fabia. least enjoyable car owned so far. :(

That looks like a thermostat housing, and the thing you have in your hand is the temp sensor. The metal thing rattling around appears to be the retaining clip for the sensor which appears to have come off.

I've stripped it all down, that picture was blatantly stolen from the 'how to replace the coolant temperature sensor' thread.

The stat is knackered, so shall replace that.

yeah, that is the stat housing... looks like the 1.4 mpi engine to me