LadyBuzz - glad to see you're going to be brave and see HR - as everyone else has said it is likely you will get drawn into it anyway and also, everyone needs to speak up when they see a bully in the workplace.
Just fyi re your concern about her treatment of you (if she stays following grievance hearing) know your rights!
There is such a thing under employment law called 'victimisation' whereby if she treats you less favourably or detrimentally because she knows/thinks you have been part of the allegations against her or given any evidence you are protected.
www.worksmart.org.uk/rights/what_is_victimisation
"Victimisation" has a special, technical, meaning in employment law. It does not have its ordinary day to day meaning of "treating someone unfairly". In an employment law context it means treating someone less favourably because that someone has exercised, or intends to exercise, rights under specific legislation (e.g. this would include raising a grievance or giving evidence to support a grievance)
This right has been strengthened under changes made by Equality Act 2010 with effect from 1st October 2010. From now on it will no longer be necessary for the victim to make a comparison with anyone else - simply being subjected to a detriment because of exercising rights under the Act will be enough to provide the victim with rights under Equality Act 2010.
I would gently let HR know you have had to find the courage to come forward, but knowing that you have the right not to be treated badly (or worse than she is already!) by this person because you support the grievance has given you a bit more strength to step up.
IME HR departments are notoriously fearful of dealing with bullies because of the fear that the bully will bring an unfair dismissal claim etc BUT if they have in mind now that in not dismissing/disciplining this woman so that her behaviour changes they are not acting to protect other employees (who will then have consequent claims against the company) they might just act. You also have additional legal protection during maternity leave and on your return to work so if you think this woman has any power to, say, make you redundant during leave, research your employment rights but hopefully she doesn't hold that kind of authority.
Good luck with HR - all you have to do is be honest and where you can be, specific. It's not a character assassination but if you can think of dates and specific acts it will be the most helpful thing you can do. Imagine coming back to a workplace were there isn't a bully! How nice would that be!