I have downloaded the ultimate British police pack for gta iv and have installed it and it crashes, I then reset the vehicles back to default game and the game and all mods run fine and all the peds and other aspects work, as soon as I add the vehicles back in though the program when I load up gta it crashes and says that it can't find a save game, this doesn't happen if I don't have the vehicles added to the .img file, am I doing something wrong? All other mods work fine I would just really like the newer police vehicles than the met police one.

Can anyone help me? 

The mods is from "Double Doppler"

Next time always backup save games even if it seems stupid... it pays!

Try getting a vanilla vehicles.img and then re adding them in.

A known issue is that some of Dave's old mods don't like LCPDFR 1.0 and above :( You should use lcpdfr 0.95 for that.

So,

This is your vehicles mods causing this issue.

You should always backup your original vehicles files when you replaced them with new vehicles mods, some of those vehicles mods does not working caused a game crash or caused any other problems, you should find another one or go back for your original files,

Also, there are quite a few vehicles mods on the website, why don't you download another one?