Hi Charlie 11,
Q.1 How do I get an NIE number and how much does it cost? You can obtain your NIE number in a Foreigner Office. Foreigner Offices can be separate buildings, or you can find them within a Police Station, as they depend on the Spanish Police. If you do it yourself, cost can vary between 9,18 euros to 16,00 euros.
Q.2 How long is it valid for? It lasts forever. At some point (say in 10 years for instance) the actual sheet might change. But your NIE number will be forever the same namber, it's like our Spanish DNI (Spanish Identity Card).
Q.3 Is it correct that when you apply for an NIE number that you are automatically on a Residency List? NO, only if you apply for Residencia then you are resident. NIE is only a fiscal number that you need to work in Spain, to buy a property, to buy a car... But you don't become a resident for having and NIE number and buying a property either. If you apply for NIE number, you only get NIE number; if you apply for Residencia, you get Residencia and NIE number at the same time, but also you have to pay taxes as Spanish Resident.
Q.4 Is a Residencia a totally differant form to a NIE and how much does it cost? Cost of Residencia is 10 euros, but again, the process is different, and not only because they are different documents, procedure is different in different Foreigner Offices, that's why you have heard some people saying that it takes only a few hours, others several weeks, for some it was a straightforward process and for others it was a nightmare, etc.
Q.5 To get Residencia do you need documentation from the UK other than your passport? It depends on the Foreigner Office where you apply for it.
Q.6 What does an NIE number on its own entitle you to? With an NIE number you can buy a property, buy a car, you can apply for a Spanish Social Security number and with both number then you can work... NIE number on its own is not a National Insurance Number.
Hope it helps,
Francisco
Sorry for my English, I use these forums to improve it