Raising a grievance at work

[kingsdk]

Hello,
I had an appraisal at work at the end of july where my manager gave me no negative feedback about myself. His comments to me were all positive and full of praise for what i have done.
Due to a different matter i asked to see copies of my appraisals and i noticed in my latest appraisal doccument my manager had added comments about me which were not discussed with me. These comments were in a negative way and i wasnt given the chance to dispute them.
Upon looking at my appraisal i noticed it has not been signed my myself and that it was signed by my manager 7-8 weeks after my appraisal took place. Some of the comments he has made hadnt even took place at the date he has signed it.
I am going to raise a grievance but what would this be classed as, is it a serious matter falsifying documents? i dont really want to get tied up in a long messy procedure at work. At the same time he needs to be punished for what he has done,
Thank you