Confusion reigns then...
I just rang the CAA and they've told me that although there is no place on the SRG1119C to 'tick' for the renewal of the IMC Rating, that I should hand write IR(R) into the space in section 6 of the form that says 'please specify including variants'.
I asked if they have any plans to communicate this information out to examiners - they don't!
Hope this information helps anyone else in this situation!