Have you downloaded all the patches off ukdirt? and installed them in the order that they've been downloaded in?
First off you should have installed Nascar Heat installed, then installed the No-Cd patch, after that installed the 1.72e patch.
After there installed you need to download the banger mod off the UK-Dirt site and install them in this order.
1st UK-Dirt Bangers Version 1.0
2nd UK-Dirt Bangers 2005 update
3rd UK-Dirt Bangers 2005 Patch 1
4th UK-Dirt Bangers 2005 Patch 2
5th UK-Dirt Bangers 2005 Patch 3
After you've done that then you download the skinpacks, again in the order they are in on the ukdirt site.
If you want real life skins then goto the skinpainting section and I think there's a sticky thread at the top of the page and it has the files for offline banger mod.
If you've had it to work before then you must of missed installing something, or done something in the wrong order.