I managed to get the old platter off - trick is to lift the motor spindle off it's bearing base with a couple of screwdrivers under each side of the platter (has about .5mm upward play). Then a light tap on the end of the actual spindle breaks the (what looks like) superglue bond between the alu platter and the motor spindle.
I was suprised before that the motor bearings felt OK after the abuse it must have suffered when someone warped the platter, but you can see from below photo that the bearing is an over engineered lump of brass, it's fine! Also, the platter itself is thing aluminium; it wouldn't take much force to bend it (although the scrathes on the CDM9 chassis tell a different story!).
As the motor and shaft seem fine, I can only assume someone thought the spin motor was faulty and was trying to remove the platter to get at the screws. I'm guessing that means there a fault elsewhere; either laser or servo board problem - does anyone know if the Naim servo board used in the CDI with CDM9 mech also suffered from electrolytic problems? There's a couple on there, they look fine but...
So, what now - old platter is junk; I am unlikely to be able to get it true again. I can't get a new one manufactured, as it has the magnet in the middle of it. I wonder if any other Philips mechs use a similar platter? I can see some begging emails to Philips coming on!
Otherwise, checking on DIY Audio, there's a whole host of other mechs I should be able to mount - CDM4/11 also has a metal spindle with magnet that I could fit to get it running (motor is better than stock, but not as good as the 9 Pro item I guess) while I search a replacement CDM4 or 9 PRO mech.
CDM4/11 is used in a wide range of old Philips machines - tracking a few on eBay now. It'll come with the servo board that should allow direct connection to the CDI.
Photo of the spindle motor removed;
http://richard-dowsett.fotopic.net/p40254164.html
Richard