Hi Fiona,

You should be looking for the same paperwork that you would expect if you were picking the car up in person: a valid NCT certificate and tax disc (check the registration on these) and the Vehicle Registration Certificate of course. If this is the first time you've seen the car, make sure you inspect it thoroughly before the delivery driver leaves. Check that the mileage is as advertised and that everything works and that there is no unexpected damage. Don't feel pressured into signing for the car until you are happy with it. 

It's also worth asking for a definition of the agreed-upon warranty in writing.