Possible slipping clutch

Evening all

My 1.9tdi superb has given me a fright on the snake pass earlier......coming up from the glossop side with a large glass aquarium in the back + 50kg of sand in the boot and the clutch started slipping a bit when i tried to overtake a slowcoach.

I didnt gear down just tried to use the torque.

Odd thing is when i got back home and emptied the car i then took it up one of sheffields steep hills and no amount of boot would make it slip.

Talking to my mate who has a volvo d5 he has been told that his clutch will allow a bit of slip to protect the gearboox it it get's overloaded.

can't see this myself but has anyone heard of this??

Failing that how hard are superb clutches to change??