£8,000 seems excessive.
To be honest, 4.25GHz on a 5960X is a pretty solid overclock. Short of binning CPUs yourself, it's the luck of the draw how high you can go. From the BIOS settings it looks like a fairly quick/dirty overclock has been applied, so you could possibly eke out a few more MHz with some tweaking - but is it really worth it, especially when you've admitted you're a beginner?
There is a little bit of voltage headroom to play with, but I'm not sure I'd want to run a chip 24/7 with much higher voltages than those. I would probably also recommend looking into how playing with such things would affect your warranty with such an expensive machine.