Delboy
Short term it is not really a problem as the over flow pipe will prevent a flood i.e. when the water in the tank reaches a cetain level, it will spill out into your garden (through the overflow pipe) rather than over the sides of the tank, Longer term it is better idea to stop the leak. Assuming that the ball cock itself is in good condition, the leak is probably caused by some grit on the threads of the connection.
When making any plumbing connections, compression joints, radiators etc, I always use a few 'winds' of PTFE tape around the threads, followed by a smear of 'Plumber's Mait' on the olive / connection. This normally stops any leak, especially if the connections are not brand new.
Hope this help
SM