So, off I goes again today up to Brent Cross. Done as everyone suggested and got the code off the £75 quid pair of jeans, Googled them and found them on some dodgy jeans site (dodgy due to the fat, hairy-belly model) for £40. I'm not sure about the postage, but that is a result in itself.
The only thing that concerns me is that the guy in the Levi's shop told me each pair is individually made and can be a slightly different fit. That means if I buy online and think they are too baggy round the arse then I'm stuck with them.
I went into Marks and found a nice pair for £30 quid. I thought they were good because of the fit, but like someone said above, loads or peeps will probably have them.