ok, I've just read the other thread and the clause. Although I know this is not what you want to hear, the clause is potentially enforceable.
there are some slight issues (i.e did you know the person when she worked at the nursery - if you didn't you'd be ok, if you did you possibly won't be)
Also when you posted the add on gumtree was it be a limited area (ie the area in which the nursery was based) If it can be said that the local gumtree ads are a good place to advertise job vacancies in the local area then posting the add could be solicitation, although that's a greyer area
so what do you do? the safest option is to talk to the nursery (or to make the offer of employment conditional on the nursery worker getting sonsent from her former employer) most nuresies would probably agree to waive the clause if you explain what happened and set out why you don't think this is solicitation (which I know goes against the opinion I just gave) but you can say that you didn't place the add there to solicit but that its a public forum and the employee just chose to apply. You should also explain that you believe its a restraint of trade as it prevents you from employing the suitable candidate from the xx that applied and prevents her from getting emploment.
A reasonable nursery may consent, BUT that clause was drafted by a lawyer and therefore they obviously had some concerns about poaching of staff. Has that been a problem in the past? If it has it will make their attitude tougher than if it hasn't.
So what will it cost, he indemnity means they probably can't stop the nanny working for you they are not looking for an injunction on the face of it just compensation. (as they are implying that they'd take damages) That would cover the costs they can directly attribute to the nanny working for you.
As I understand it she had already resigned. Is that right? If so and she worked her notice then there are no losses for the nursery. If on the other hand she resigned to work for you then she/you may be liable for any recruitment costs they have advertising agenices , also is she didn't work her notice there may be costs asscoiated with covering that period.
I would add that it is very rare for companies to enforce these clauses as the law is very complex in this area, and its very expensive so usually a deal is done.