Redex might help, but one of those treatments for about £10 - £15 from Halfords & 'an Italian Tune up', just before it goes in to be re-tested (i.e. a good run at motorway speeds (at 70mph, or at least above 50mph so the engine / exhaust get nice n warm), for about 20 - 30 miles / 30 minutes), can make all the difference...
...That said, there's no guarantees... !
...That said, there's no guarantees... !