Thank you for your question. There is no time limit on raising a grievance.
If you have not dismissed her you will have to follow the standard grievance procedure (not the modified version):-
Step 1- the employee must submit a written statement of their grievance. It does not need to be too formal, all that is required is that the employer can understand the general nature of the complaint being made
Step 2 - the meeting - the employer must have been told the basis for the grievance and had a reasonable opportunity to consider its response. There should be an open discussion of the issue. If there there are no management persons that aren't include in the grievance then you are obviously frustrated from having someone there who isn't involved. Consider having an adjourned meeting in which you give your decision.
Step 3- appeal -Another meeting on an appeal basis If so required by the employee. You should have a manager there more senior to the one at step 2 if possible
It is important that you make them aware of the grievance procedure, that they can be accompanied to any meeting, that each step is taken without unreasonable delay, that the time and location of meetings are reasonable and that both sides are afforded the opportunity to explain their case.
ACAS can offer you free advice, you can call them on 08457 47 47 47.
If this is useful please kindly click accept so that I may be rewarded for my time. It will be gratefully received and you will be free to ask follow up questions.