Zedder,
Just reading this:
Pensions increase - HM Treasury
it looks like it is RPI until Apr 2011 (i.e. Sep 2010 RPI figure) at which point CPI takes over. But the bad news is that Sep 2009 RPI was negative, so zero increase was applied - so I don't think you PA level 31 calculation holds....
About to get out my own calculator and do my own sums, but I'm no further ahead than you yet....