Oh that's a shame love, I hope it's all sorted out soon.
In my (limited) experience of this sort of thing there is usually a set time frame with which the investigation is carried out and then a meeting is arranged to discuss what happened, what your take on it is etc... A report is produced and then if any other formal actions are going to be taken (discplinary/grievance) then you have to go to another meeting. From what I remember, you can have someone with you at both these things. So I think the long and short of my answer is yes, they can investigate without you there as they'll probably have to do that initial part within a set time frame.
Do you have a copy of your disciplinary/grievance procedure as well as a copy of the complaints procedure? Having a read through those might help you anticipate any nasty suprises.