you will need to complete all challenges at platinum level to unlock the TVR 12 speed. Gold unlocks the Merc and silver the porsche 911 GT amongst others (correct me if i am wrong). I have done all at gold and 60 at platinum before i gave up because they were getting to tough. I now play mostly on live.
Thanks Steve apart from the final race which i am trying on gold i have nine silvers to try and get into gold to unlock the mercedes, most of these are the hot lap which i just dont seem to be getting anywhere near the time, unless i must be using the wrong car
i can most things on gold or platinum but i cant do a lot of the cone challenges on platinum. so i was forced to (im ashamed to admit it) get a gamesave with all the cars unlocked so that i could race the speed 12. some of the platinums on this game are astoundingly hard. loadsa people who have tvr's on live use the cheat, infact im pretty sure its close to 80%.
if you decide to take the plunge and cheat have a look at www.xbox-saves.com