I am thinking qf purchasing a Nikon 24-70mm f2.8 which costs over Â£1000 but the same lens from Sigma costs just over £400 why is there such a big difference in price is there much difference in the glass used
Sigma currently list 2 24-70 f/2.8 lenses, 24-70mm f/2.8 EX DG HSM and 24-70mm f/2.8 EX DG MACRO. The 2nd of these is about £430, and I guess that's the one you were looking at. The first is £730 and is probably more comparable to the Nikkor. The two Sigmas have different optical formulas, so will perform differently.
I think it's about normal for a Nikkor to be 50% more expensive than a 3rd party lens. It's not always very much better, though.