I made a cheap, easy bearing retainer removal tool out of a piece of steel flat bar and 2 short 4mm bolts.
Just drill 2 holes in the bar the same distance apart as the holes in the retainer, tap the holes 4mm and screw in the 2 bolts. If you don't have any taps you could drill clearance holes for the bolts and use a nut to hold them in position, just use slightly longer bolts.
They should locate in a pair of holes in the retainer and unscrew it nicely. Mine came out no problem, no damage and I'll keep the tool handy for future use.
If you've got a really stubborn one(oo-err!) and worried about damaging the retainer it would be quite easy to make a wider one and fit 4 bolts to fit all 4 holes on the retainer.
If that doesn't work then by the time you get the retainer out it will be scrap. New ones are available and quite cheap.
