It's �19 for a replacement Registration Document ( Log Book) from the DVLA. Do it over the phone by credit/debit card. Not sure if you can do it on line.
In addition to Normans response, its also on the front and back of the car :-) presumably you mean get a replacement log book by providing the registration number - you need a form V62 and it looks like it cosrs �19 http://www.dvla.gov.uk/forms.aspx