Your annual pension statement whether you're a current or deferred member will be dated 31st March, therefore the inflation figures used to determine the annual increase will be those from September 2018(released October 2018).
So it shouldn't be too difficult to find out what those figures were and do the maths yourself.
If you're in section C and are a deferred member your pre 1st April 2018 pension will actually be going up by CPI. Details here
If you're in section C and are a current employee it'll be increasing by RPI. Details in your plan guide.
But if you've still employed by RM and opted out of the DBCBS, it will be CPI. Details here