Thank you all for the great feedback. Unfortunately I am probably still as confused as ever.
I am quite aware that I have to act on this but my biggest concern is that my actions could be construed as constructive dismissal. E.G. If I bring her in for a chat and tell her that I have 5 complaints against her. I might as well be saying that everyone hates her. She then decides that she can't work here anymore and it ends up costing me alot of money.
My feeling at the moment is that I should bring her in for a chat along the lines that "I" have noticed a bad atmosphere in the department and want to clear the air. "I can see that certain employees seem to be under pressure"
If that does not work then my next action will be to bring in professional help.
Putting actual complaints to someone that you don't believe are frivolous will not support a constructive dismissal claim. Put specific examples that people have told you to her, and ask her for her side of the story. This first step is an investigation into whether the claims can be substantiated, not to present her with the fact that no-one likes her. This is a diplomatic process, to be sure, but one where, if it's done properly, works all round and will not leave you with any exposure to liability. In fact, being more general about the situation would be less constructive than actually telling her of the specific circumstances and behaviour that have been the subject of a complaint.
If you don't want to discuss the specific circumstances, I would advise against the type of meeting you propose and bring in outside help straight away. I'd probably use a HR consultant for this. I've been lucky to work with some great HR people and, when something like this is done properly, it really is an admirable thing to see. Your brief to the outside help will be important and you should document what information you have given them - you do not want to pre-empt the conclusions of the investigation.
A more economic alternative is to sit down yourself with a HR consultant or HR lawyer and get some advice as to how to handle this yourself. I can imagine that this is causing you stress, as well as your staff and, the sooner you deal with this issue, the sooner all can get back to work without this cloud hanging over you all.
For what it's worth, I think you should meet with her and say that there have been a number of complaints about her behaviour that some have felt bullied or undermined on those occasions. Explain to her that, because of the importance of the bullying policy in the workplace, you have to investigate these claims. Then put the specific circumstances of each claim to her and ask her for her interpretation of what happened. Say that you are aware that there are two sides to each story and that she is there to give you her side, but that you are under a duty to investigate the claims since they were brought to you. Until things are sorted out, ask her not to discuss the claims with anyone else. Once this initial meeting has been held, you will have more information at hand to see if the claims are valid and if they should be acted upon. Don't forget that, even if the claims are substantiated, it may not be enough to dismiss her, so approach this with a view to making everyone happy, rather than as the start of a process to fire her. It may be that she just needs to see that specific behaviour is not acceptable and that she will work on improving her attitude. Make it clear to her that it is *you* who is demanding a change in attitude (rather than her workmates) and that she is responsible to you. Deflect her attention away from viewing her workmates as being the antagonists and make it between you (her manager) and her.
Sorry for the long post - it's difficult to give proper advice over an internet forum and ultimately it may be better for you to call someone to chat through things properly before you sit down at a meeting with her. It's good to work out a script beforehand so that you pitch the problem in the right way.
Good luck
Sprite