Do VW Polo 9N springs (front + rear) fit on a mk 1 Fabia ?

Providing they are from the same spec, ie 1.2 petrol polo springs on a 1.2 Fabia etc then yes

So basically what you are saying is that they are physically different (different engine types, different mounts) and they would not fit or they have different specs ?

Different Spring rates for different engined models

Ok, I've understood that, but do they actually physically fit ?

Yes they physically fit but fitting tdi front springs on an mpi would lift it up by around an inch and same with estate rears.

I used Vrs springs on our endurance 1.4 16v to gain a bit of clearance

Did you fit tdi shocks as well or only springs ?

Yes shocks as well but the springs all fit any of the shocks.

My H & R'S came off a polo

I've found a set of Eibach Pro Kit for VW Polo 9N for all engine types which are about 30% cheaper than the Fabia MK1 ones.