For what its worth...
I bought an 'n' reg 1.4sx 60,000 mile Brava 3 weeks ago & since then have had to replace the clutch (£260) from auto-fit centre. One garage I spoke to said he'd just replaced the clutch on another 60,000 mile one (gave mileage without any prompts from me) so maybe this is not unusual. My clutch wasn't slipping but the thrust bearing was gone & was squealing. Maybe yours is an engine out job since its £600, otherwise seems expensive (you have to drop the transmission on my car & they charged 4 hours labour for this).
If your car is under warranty I feel they should replace it free of charge. My other car an Audi A4 (bought at 3 year old 58,000 miles from an Audi dealer) had transmission slack. They kept assuring me it was normal But I kept telling them it was not right. As an act of good faith they replaced the clutch, gearbox & differential. The car has still transmission slack but I have since learned this is normal & indeed it hasn't changed in 35,000 miles.
Good luck, you shouldn't have to pay, you are an unhappy customer & they should value repeat custom.