You do not say if this employee is employed under UK employment law or Dutch employment law about which I have no knowledge, assuming it is UK employment I would suggest that appearing too be dissatisfied with the conduct and/or capability of this employee, which are both potentially fair reasons for dismissal, you consider dismissal or disciplinary action. If there are no terms which provide the employee protection against you taking action on the grounds you have in mind in their contract of employment you should write to them setting out the circumstances which lead you to contemplate dismissing or taking disciplinary action against them and invite them to a meeting to discuss your concerns after they have had the opportunity to consider their response to the matters you raise. At the meeting investigate why the quality of their work is poor and ask why they take so much sick leave, there may be a good reason. After the meeting inform the employee of your decision in writing and inform them of their right to appeal, if they choose to appeal a further meeting should be held but this meeting need not be before the dismissal or disciplinary action takes effect.
The employee requires one year’s service to take the matter to an employment tribunal and you are under no obligation to make payments in advance..