I spoke to Platinum customer service today. They said that Centurion look mainly at two things: (1) how long you have been a Platinum member and (2) your spend on Platinum (not any other Amex credit card).

They said that you need to demonstrate a significant monthly spend on Platinum only for at least 12 consecutive months to qualify for consideration for Centurion. They wouldn't even register my interest without this prerequisite.

Seems a bit daft but that's what they insisted on. I'll try switching to the Platinum card but I disappointed not to be getting the Amex 2-4-1 coupon deal nor BA Miles. Stuck with Membership Rewards but I guess I can always go back to spending on BA-Amex once I get Centurion (2006-2007?). I hear that the BA-Amex relationship is up for review next year in any case.....