Case law in the last few years has made the position on prior final warnings very clear. The test is whether it was reasonable for the employer to treat the misconduct, taken together with the warning (assuming it was issued in good faith, not obviously inappropriate and that there had been, at least on the face of it, grounds for imposing it) as sufficient to dismiss. By the same token, it does not matter if the offence for which the warning was issued was for conduct of a different nature; the question is simply whether the employer was entitled to rely on the warning.
In Trye v UKME (UK Mission Enterprise Ltd), an employee was given a final written warning after various misconduct allegations of failing to follow reasonable instructions and acting in a way that brought the company into disrepute were upheld. A year later she was dismissed following an investigation into her failure to comply with the company's absence procedures, having been given a specific warning a few months before about the need to follow the absence reporting procedure. The Employment Tribunal and EAT decided that her dismissal was not unfair.
The EAT said that if there is a final warning that is current, it is only in exceptional circumstances that further misconduct will not be met with dismissal, regardless of whether that further conduct appeared to be serious enough in itself to warrant dismissal, adding that even an expired final warning can be relevant to whether a dismissal is reasonable.