remap will give slightly better economy. How many miles are you doing on the £20, it may not last you a week, but e.g. your mate could be doing 50 miles a week in his AUM, but you might be doing 50 miles a day, so obviously your going to get through more fuel than him. If your doing similar mileage, you should get it checked out. Use a fault code reader to see if anything comes up, if you want one, get a U280 or U281 off ebay and see what happens (y)