BP ultimate is very good indeed, and the performance gain can actually be felt, however because I use old school diesels I have other problems to contend with.
Crap quality diesel isn't one of them because it will run on anything, even parafin or engine oil, clean or dirty (after it's had the bits of crud filtered out) however the problem with diesel for new engines is the lubricating properties, yes it has better lubricating properties than petrol but still not the best for old injector pumps.
Since the EU said all diesel must now have a low sulphur content (which is the main lubricant) but you can just add used engine oil, clean mineral oil or 2 stroke oil or even veg oil to compensate.
I can tell you catagoricly that the raw product (the diesel itself) is the same no matter where you buy it, it's the additives that very from company to company supermarkets just have the raw product and is cheaper, because they don't spend any money on R&D for additives.