Subform Link Criteria

[jax]

I will try to keep this as simple as possible. I have a main form witch lists premises details. I have a subform which is displayed as a list of rooms on the main form. To edit or add information about rooms and users I have a command button opening an "edit subform". The mainform and both subforms are linked by a premises id and the name of the premises is displayed on each form. What I want is for the user to select a room from the list on the main form/subform and when they click the command button the "edit subform" opens at the relevant room. At the moment the edit subform opens at "A" in the list. I have the criteria of the command button set to link the ID of the premises. I hope this is clear and someone can help me? Thanks