Here is the legal position on warnings:
Warnings seem particularly appropriate to cases of misconduct by the employee, but the general requirement for them has also been applied to cases of lack of capicity, such as, inefficiency, bad workmanship and incompetence. In line with the accepted principle discipline should be constructive as well as punitive.
Many employers have a definite warning procedure built into their disciplinary code or have agreed on the basis of collective agreements. You need to follow these procedures in your own company or else it may result in labour disputes.
Often it is the practice to give informal oral warnings for the purpose of improving conduct when employees commit minor infringements of the established standards, However if it is of a more serious nature written warnings are the recommendation.
Just to summarise:
In case of minor offences oral warning is ok. For serious offences you give written warning.
Further misconduct might warrent a final warning,
The presence or absence may be the most important factor in detremining fairness or otherwise of a dismissal. Or else the court or tribunal may think in the absence of a warning - that it was not important anyway.
The question of whether the employee is permanent is another matter. He is still on a contract of service and not a contract for service. Even though the employer could dismiss an employee during probation you need toi show that it was fair. An occasional warning may not appear to be fair before dismissal.
Trust this clarifies.