has any1 ever used a roasting tin to bake a cake in? i know i would have to lin it with baking paper and probably use 2 packets of cake mix? how long would it take to bake and at what temperature?

Not in a roasting tin as such, but in plenty of strangely-shaped pans! It should work just fine, but you might want to consider using a baking core or a flower nail (or maybe 2 or 3) to make sure the cake gets done in the middle. Since roasting pans vary in size, the amount of batter you would need would depend on this. One way to find out is to put water in the pan 2/3 of the way up (or however high you want it to be filled with cake batter) and then measure this water cup-by-cup. That's how many cups of batter you'll need, and you can (hopefully) extrapolate that to the number of packages of cake mix you need.

thanks thats a great idea? do u know what temp i should have it on?

I generally bake larger cakes at 325-350°F. Some experimentation may be necessary.