Didn't want to hijack the bumper clearance thread...

i was just wondering what the purpose of the fin under the front of the fabia is.

Was thinking it could produce negative lift, but then just laughed because that seems a bit far fetched

Was wondering that myself, have seen similar on Saabs, but behind the front wheels, like a small mudguard halfway along the underside of the body...

I think it's to prevent spray and crap being slung forward under the car, assuming you mean the mini-flaps in front of the wheels.

There are ones at the front of the wheel arches but then one long fin along the length of the car between the front wheels

Do you mean the wind deflector/undertray under the car? - Stops road crap going up into the engine bay and guards the underbody.

It's incase of a "Jason" that way the car looks cool underneath!