You also need to apply for a temp airmans cert for the IR (and the twin if you have it) to add onto the existing TAC you have for your PPL.
Apply for this via the standard route (pay the CAA £15) pass the computer ground exam and go to the FSDO to pick up the cert.
I don't believe any flying is required.
I tried to do it last year but the new TAC process was in turmoil. I'll have to do it next time.
Good luck