There is an implied term in employment contracts that the employer shall render reasonable support to an employee to ensure that the employee can carry out the duties of his job without harassment and disruption by fellow workers.
As always, the position in any particular case will depend on the facts. It is not every course of victimisation or bullying by fellow employees which will give rise to a cause of action against the employer, and an employee may have to accept some degree of unpleasantness from fellow workers. Moreover, the employer will not be liable unless he knows or ought to know that the harassment is taking place and fails to take reasonable steps to prevent it.
Acas produce a guide on bullying and harassment with useful advice to employers on the management of the problem and a separate guide for employees. Most employers now will have formal policies on bullying and harassment and if you feel you are being bullied you should follow the mechanisms of the policy and make it clear that you expect them to be honoured. However, do remember that behaviour you regard as bullying may not be regarded as such by your colleague and so you may be able to resolve the problem informally by making your perceptions known to your colleague.
Employers have an obligation to offer a safe place of work and an assessment of the stress risk is something all employers should do.