A landlord should first attempt to communicate with the tenant to resolve the issue. Should the non-payment continue, issue formal written requests for payment. Should the rent continue to be unpaid, consider taking legal action.
If the amount of arrears is less than £10,000 you can issue proceedings against the tenant in the Small Claims Court. If the tenant admits the claim there will be no hearing. However, if they do not there will be a court hearing and the matter will be dealt with by way of negotiation.
If the amount of arrears is more than £10,000 you must issue proceedings in the County Court. It is advisable to seek the appropriate legal advice from a solicitor or the Citizens Advice. The failure to pay rent does not give the landlord an automatic right to evict the tenant. If the correct procedures are not followed, then the landlord may be liable for harassing or illegally evicting the tenant, please see the related question, Q47 (What is the correct procedure a landlord should follow to legally evict a tenant?), for the proper procedure for eviction.