I would say that a professional nanny should have relevant qualifications. While I appreciate that there are many nannies out there who do not have any qualifications, I do feel that by making the commitment to training, a qualified nanny is in a better position to provide care for pre-school child.
Qualifications wise, there are many available. I would suggest looking at the list of Level 3 qualifications available at Children's Workforce Development Council
By referring to the list of qualifications that Ofsted use as guidance for nursery room leaders, that with luck will result in you selecting someone with an understanding of child development and pre-school education. It won't alas guarantee that they know how to, or have ever changed a nappy!
What are these qualifications? Actual course content will vary from course to course. There is criteria set down for specific levels of courses, so this Level 3 Criteria may help give an idea of the type of thing that is covered in the course.
CACHE I feel are the largest examination board dealing with Childcare courses. Their website CACHE.org.uk gives information about their courses - the link in this case goes to Level 3 Diploma in Childcare and Education, which would be most similar to the older NNEB I feel.
So in answer to your question: Is there a common recognised qualification for a nanny or just a mish-mash - eg NVQs, First Aid etc?
No there isn't really... once upon a time there was the NNEB, but now it is a mish-mash of NVQs.
First Aid however is separate to childcare and education qualifications. It is important for anyone caring for children (including parents) to know basic First Aid as children can be accident prone. There are alas several different courses available, so varying levels of compentancy. However, some training is better than none, so I wouldn't worry too much about the level of First Aid training, but more about if it covered Infant Casultaties and how recent the training was. Ideally it should be repeated every 3 years. If you intend to benefit from Childcare Vouchers, then your nanny will need to have done a First Aid course that meets criteria set out by CAS - First Aid criteria
Hope that is of help.