What happens is the seal at the gearbox end fails, contanimating the clutch plates. By the time they start slipping it's too late, you need new clutch plates as well as a fix. Once fixed it should be okay. Mine failed at 18k miles (less than a year's worth of riding for me so under warrantee) and was fixed, since then I'm up to 57k miles now (so another 39k miles on the new clutch) without a problem.
I have to get an MoT in July, first time I've had to do that in a while.