I've just bought a set of aftermarket headlight units for the car, but slightly concerned that the units will be for a LHD car, rather than a RHD car. I did a search but could only find reference that LHD/RHD use generic clusters, is this correct? 

The rear clusters are indeed generic, however the headlamps are not.

LHD drive headlight units have part numbers beginning 6Y1, RHD begin 6Y2; look for what your aftermarket ones claim part number equivalence to.

https://skoda.7zap.com/en/cz/fabia/fab/2005-453/9/941-941000/