Turn the flywheel over and use two, (or more), of the eight screws which held the alternator rotor to the flywheel to push the flange out (don't screw them all the way in of course - just locate the threads into the flange). You can encourage them with a soft mallet if you support the flywheel edges and tap them alternately.
This from the workshop manual:
"
Insert two of the bolts (25) just removed from flywheel rotor-side in their holes in order to remove the flange (13) and the starter clutch (21).
The starter clutch is a slight interference fit on the flange. To remove it, use a suitable drift.
Edit I've also included the refitting instructions for completeness as they're quite helpful.
When refitting the flange (13) to the flywheel (16), position it with the tapered side facing the flywheel.
When refitting the flange to flywheel, insert a pin or a wrench inside the hole for the rotor flywheel locating dowel (14) in order to line up flywheel holes with flange. This operation proves useful since the hole cannot be aligned after having fitted the flange to the flywheel due to the interference fit of this part.
Position the locating dowel (14) with the circlip (15) on the flywheel.
Apply threadlocker to the screws (25) and fix the rotor (23), the flange (13) and the starter clutch (21) to the flywheel (16).
