Unfortunately I see the NannyJob specimen contract to be a little lacking when it comes to this particular clause. I feel a Gross Misconduct clause should be very much an Open and Shut case - it should list explicitly the actions that would result in instant dismissal, as there would be few of them.
However, that is just my view, so if you were to use that clause to dismiss someone, I would suggest contacting a lawyer as the clause does allow you to add things to it, but doesn't tell the employee in advance what those things are.
I feel you would certainly be able to issue a written warning, but going straight to dismissal could if the nanny challenged it, cause problems, as the contract does not state (in my view) sufficient examples of what consitutes gross misconduct. Therefore another option open to you would be to dismiss on notice. More info about Managing Misconduct
The DTI Dismissal - fair and unfair booklet may also be of help, in particular the bit titled: What is reasonable procedure and sufficient reason for dismissing an employee?
Located within the DTI website, I found an interesting document which mentions Gross Misconduct. file23903
It is a consultation document regarding Employment Act 2002. It says "if the employee complains to an employment tribunal, it will almost always be held to be unfair dismissal if the employer has dismissed immediately on or after becoming aware of the act of gross misconduct and without first investigating the circumstances".
Therefore, in situations of gross misconduct, procedures need to be followed, in terms of investigation of what happened, during which time the employee can be suspended on full pay.
To conclude, if the contracts states the reasons under which the employee can be instantly dismissed, then dismissal I would consider is reasonable. When however, it is left open as in the case of the contract in question, I feel that pushing for gross misconduct would be a mistake, as it could too easily be challenged. It would however warrant a written warning, and perhaps a review of the contract in which it is added to the gross misconduct section so that if it happened again, it would be instant dismissal.