if the nail is in the tread contact part it can be repaired quite safely. it cannot if it is in the wall. go to a reputable tyre dealer who does repairs not one trying to make their bonus. usually a small co, it will cost £20 for repair an balance.
if you go to google an type in your tyre size it will come up with lots of deals. makro is one if you know someone who has membership. good luck i have had 3 nails in the rears up to now 2 years.