I get the BG244 off ebay (whoever is cheapest) and with the Millers usually OPIE. The last load of Milllers was a couple of years ago (ecomax but was called diesel power sports before then). It usually costs about £65-£75 but lasts for about three years feeding all three cars at three fill ups a month. For the Leon the 5 litres should do 100 fill ups.
I would really recommend the Millers and have been running on it for years. In simplistic terms I believe it transforms stock Shell into some kind of V power variant with all the benefits (less smoke, cleaner running, a bit more mpg etc) but is more cost effective. At 10p a litre premium for V max on a 100 fill ups this costs £500 (50 litres a go). The equivalent Millers additive = £75 say. No brainer imo.
As said on the BG244 earlier, I may be overkilling (wasting money) because of the Millers, but that's the worse it can be and as an annual cost (I think from memory people who use BG244 regularly) tends to be just the once a year so hardly the end of the world.
GL with the motor