Has anyone replaced there standard steering wheel for the Flat bottomed Steering Wheel, if so how big a job is it and any info on step by step procedure

 

See this recent post by xdave: https://www.jaguarforums.com/forum/f...8/#post2162807
Not done it myself but the same upgrade as you are after is something I have been chasing for years, just need to find a good condition used flat bottomed steering wheel going cheapish as the new ones cost a bundle (especially the heated version like I have).
It doesn't look all that difficult to do and apparently the hardest part is unhooking the two retaining clips without the special tool.
And to remember to unhook those two clips first then disconnect the battery then remove the air bag.

 

Aside from the looks, is there any advantage to fitting the flat bottomed wheel?

The 'round' wheel is off-center, biased upward by design, and it has been fine for us (as far as entry/exit and view of the instruments).

 

For those with longer legs, it helps keep the wheel out of the way of their legs/knees. I'm not referring to just entering and existing the vehicle in which most modern upper tier cars auto raise and lower the wheel. I'm more referring when when driving. If you've never experienced this issue, you likely don't have legs long enough to experience it.