Louise, you need proper expert advice about this, not random thoughts from (probably ill-informed) strangers on websites. In your place I would get further legal advice from someone who can walk you through the steps you need to take. Your estate agent may be wrong about needing to contact the woman whose payments you've guaranteed, which a lawyer could tell you, but even if it turns out s/he is correct, you could get legal advice about how to try to contact her - it might be that you've just got to do certain things and if she doesn't respond to them then you can move to change the guarantor without her agreement. But you won't know any of this without getting expert advice.
There are lawyers who work pro-bono (free!) so you could google 'pro-bono law' + your town and see if you can find any that way; alternatively try the citizen's advice bureau; if you are a student the student's union might be able to offer free legal advice; if you aren't a student but live near a university where law is taught, you might find the university offers free legal advice (given by students under the supervision of highly-trained lawyers). Or you might find that your house insurance or your parents' house insurance includes free legal advice (though whether this would be covered is doubtful perhaps).
Another thing I would do in your place - actually, the first thing - is to get out the guarantee you signed and read it carefully (and make sure you take it along to your lawyer) to check what you have actually signed up to. Perhaps you only agreed to guarantee the rent for a certain length of time, and that time has now passed or will come to an end soon?
Also, I would be extremely reluctant to have my father take over the guarantee, and especially for a woman who is clearly a bad risk (not to mention undeserving). You could try just telling the landlord in writing that you are no longer in a position to guarantee the rent and will not do so (and then let him or her know that your Dad won't be taking it on either). Of course the landlord might still have a right against you for the rent if it isn't paid, but if you are badly off yourself and have no assets, and can make this clear to the landlord, then the landlord might just think there is no point in having all the hassle and cost of trying to take you to court and it would be preferable to ease the tenant out instead. Until recently I was a landlord, and that's the view I would probably take in a situation like this.