There may be a provision in the lease that obliges a guarantor to pay any sums due if a tenant does not pay the rent. Similarly, you may be able to recover rent arrears from ex tenants if they have guaranteed the obligations of the current tenant by assigning the tenancy.
Finally, you may be able to recover unpaid rent from a sub-tenant (the tenant of your tenant) in some circumstances, subject to the terms set out in the Landlord and Tenant (Covenants) Act 1995.