Can an employer and/or its employees be liable for harassment on the grounds of religion or belief where the victim is mistakenly believed to be of a particular religion or belief?

Yes. Section 26 of the Equality Act 2010 prohibits harassment that is "related to" the protected characteristic of religion or belief. This wide definition means that the harassment does not have to be "directed at" the actual religion or belief of the victim. It could relate to a religion or belief that they are, rightly or wrongly, perceived to hold. Even if the unwanted conduct concerns a religion or belief that the victim is not thought to hold, this would still constitute harassment "related to" religion or belief. The employer will be liable for such conduct unless it has a reasonable steps defence. The individual employees who perpetrate the harassment will also be personally liable under s.110 of the Act, irrespective of whether or not the employer has a defence.