Car radiator leaks can happen at any time but it is important that you spot them quickly to avoid the car verheating and causing significant damage. It can be due to damage to your radiator caused by a bird flying into it, a stone hitting it or just due to age and deterioration. The leak may only be small or may be a huge hole causing you to lose fluid nearly as quickly as you put it in.
If the leak is small there are a number of things you can try to fix it before having to fork out the hundreds of pounds a garage will want to supply and fit a new one. There are a number of products such as RadWeld that you pour into your coolant that will circulate around the car cooling system, finding any leaks and plugging them up. The solution will set in these holes and form a bond to keep the coolant in. This can be a permanent solution depending on how bad the leak is and how well the product works. These types of products do have very good reviews so I would definitely suggest using one before going to a garage as the solution only cost about eight pounds.