Do you know of anyone else at your workplace that has complained about similar events, maybe even regarding the same people? This can be excellent evidence of failures by your employer that will further strengthen your case. The employer has a responsibility through laws to do whatever is reasonable to keep you safe at work. A failure to act on previous concerns is a great indicator that the employer has failed in its responsibilities.
Under the Health and Safety at Work Act 1974 Section 2 (1)(2)(a)(e), the employer is made to ensure that the "...working environment for his employees that is, so far as is reasonably practicable, safe, without risks to health". Section 109 of the Equality Act 2010 also clarifies that the employer has this responsibility when it comes to discrimination, harassment and disability and that they might be made responsible for acts of their employees (i.e. harassment) even though the employer has not authorised it etc.