If the boots look OK and are not cracked then to cured the problem replace the boot clamps that are certainly too loose. You can back off the boots while there and repack with some grease.
Look at this thread :
Axle boot clamp question
An explanation of the sudden leak is probably due to the sustained heat from a long 1500 miles drive. I bet it's been a while since the car was used so long and intensively ?