As far as the Crown Prosecution Service is concerned "serious injuries include damaged teeth or bones, extensive and severe bruising, cuts requiring suturing and those that result in loss of consciousness." (Link as above).
Statements by the CPS aren't law but, because they're based upon their experiences of prosecuting hundreds of thousands of assault cases, they're still a damned good indication of how a court might interpret the law.
So, if a victim required stitches, it's likely that a court would regard that as 'serious injury'.