If the Tenant does not pay the rent, the Tenant is in breach of contract (written or verbal).
Practically the first step of action is to make contact with the Tenant to establish the reason for non-payment. You will know your Tenant and be able to assess if this is a once-off situation or if the Tenant routinely pays late or only partially.
In terms of the Consumer Protection Act, the Landlord must demand payment in writing and give the tenant 20 business days to settle.
Should the Tenant fail to settle after the demand, the Landlord is entitled to cancel the lease and demand the tenant vacates the property.
Should the Tenant fail to vacate the property as requested, then the Landlord must take legal action to obtain a court order Eviction, after which the Sheriff of the Court will forcible remove the tenant.