I don't believe a graduate tax is a fair way to to solve this problem. In my opnion a universtiy degree is essentially a product, and should have a fixed price, not one that varies with your success. The problem is at the other end of the system. I payed £1300 a year my 4 years getting a masters in physics. I had access to fantastic labs and workshops with decent lasers, nano-material fabrication, decent telescopes. Hell I got to make my very own superconductor. On top of this I had roughly 30 contact hours of tuition a week. Whereas a student who studied English would have payed the same amount, have gotten none of their coursebooks included in the price, and would have had a maximum of maybe 12 hours contact with tutors a week.
Either they were extorted or I got a bargain. This difference in what you pay/what you get is still there with this system. I think this is where the system should be shaken up - make us contribute in proportion to what it costs to teach us, rather then what can be squeezed out of us.