Engine/gearbox software upgrade?

I have recently bought a 2004 (54 plate) 2.5 SE auto saloon, and have read (albeit on another U.S. forum, dated 2002) about a software upgrade to the engine/gearbox management system, from Version 16 to Version 20. Apparently this produces a much smoother shift between gears, and slightly increased performance. My car certainly seems to have 'strange' shift points, with slow pick-up, almost feeling like a manual change at times, it's so rough. For instance, when I'm decelerating and reach 15mph, the gearbox noticeably changes down, and it almost feels like I've momentarily applied the breaks, it's that jerky!

Is anyone familiar with this upgrade? Is it still available? Would Jag main dealers know what I was talking about if I discussed it with them?

Thanks for any comments....