It may well be that your HR dept has not acted on it as you were signed off with stress, and then away on maternity leave. I suspect in these circumstances ,they were being wary because had they done anything you could have alleged they were hounding you whilst you were off with stress.
You also did not answer my question about your performance; despite your manager taking a bullying approach according to your allegation. Was it substandard and were you off because you could see a disciplinary interview coming up? This may not be the case, but if it was, you would not be the first person to duck out of the way like this and use your absence to avoid the issue.
In the circumstances, I do not think you have any choice but to face up to it and as others have said go in for the grievance hearing, and take a representative with you. You say it is with the manager's "best friend", but that an HR rep will be there. The Company is entitled to nominate whoever they want to deal with this and you need to consult the company's grievance procedure to see what the next stage is after this, if you are not satisfied with the decision reached. You have better face up also that they may be two issues here, firstly your grievance about being bullied by the manager and if you can prove this, and secondly, there may be a separate issue around your performance. This should be raised as a separate matter by the company.