There is a huge difference between a Nanny and an Au pair. Au pairs usually are on very basic money - not much more than pocket money really and you are responsible for providing board and lodging for them. They typically can only work 25 hours per week and if you get a very young one (16) it can feel like having another child rather than child help. There are very good websites where you can exchange emails and contact information with prospective Au pairs before committing but the majority do come from Eastern Europe. Some very good friends have had about 5 different au pairs, no disastrous one's but some were easier than others. The Hungarian girl was a stickler for time keeping and only worked the agreed number of hours and no more and ate like a horse. Another was a 16 year old Blond Swedish girl (settle down at the back there!) but was like a child really although very pleasant but could not help very much. One Slovakian girl was a hero, she could drive and had a boyfriend in the UK and would work loads of extra hours, was really friendly etc. Like in real life people are different.
Nannys are self employed girls and it is your responsibility to check they are qualified and CRB checked etc. There salaries are usually quoted NET at a daily rate of somewhere between 60 and 100 pounds per day depending on experience and location. Normally live out these days but a few live in jobs are mainly in London. Access to a car is expected or alternatively petrol money is paid. As they are self employed you are responsible for their PAYE tax and unless you are an expert it is worth paying a company like Nanny Tax (200 per year) to administer the pay slips etc. In short a full time Nanny will cost you their Net pay 15.6K to 26K per year plus their Tax and NI plus employers NI contributions, Nanny Tax Fees, Car use/Petrol money and it is an unwritten rule that you remember their birthdays and can bring them on holiday with you (paid of course). They get 4 paid weeks of holiday a year, sick leave (at least the government pays the SSP) and they can get pregnant as well! Which means you need cover for 6 months or so (this happened to us twice).
In short, nannies are a fantastic way to ensure kids get top quality care (if you get a good nanny) and worry free cover but they are extremely expensive, you would pay a minimum of 20K out of your NET salary and it could be as high as 35K per year. You need to be earning significantly above those levels to make it worthwhile. At least the cost stays the same no matter how many children you have but nursery costs are a fair bit lower.
The most cost effective childcare method is a registered childminder. They are local authority registered and regularly checked people (usually mums with older children) but will typically require you to bring your kids to them and pick them up. Some will take kids to after school activities but it will depend on their circumstances. They normally charge an hourly rate per child, somewhere around 5-9 pounds per child per hour.
Hope this helps, and feel free to ask any questions. We have had (part time) nannys for over 10 years and they work very well for us with my wife working 3 days a week. Many of our local friend use child minders and if we could find one that would work in our situation we would go that route. However as we can't the Nanny is the most flexible solution.