It's not really worth bothering for less than 75% these days, is it?
Someone earlier was mentioning that they saw different prices with the new site. I've just been given three different prices trying to buy a book with the 85Off coupon. On this machine, using an oldish version of Safari and the old site, it shows as £10.31. On my iPad, using iOS6 and mobile Safari and the new site, it shows as £9.71, and then when I actually go to buy it, it shows as £8.33.
Actually, I've just realised that I have been charged based on £9.71 so I guess that last price must be less tax. The way it shows the discount is bizarre, since it's the difference between the tax-free full price and the tax-inclusive discounted price, which is not 85% of anything relevant, but it comes up with the right answer so I guess it doesn't matter in the end.