Adjusting the baking time for large cakes?
The recipe should specify the temperature, as it can vary depending on the ingredients and the desired texture. If I had to guess, I'd use a temperature of 180°C (350°F), or gas mark 4. However, some recipes call for 3 or 5.  (Growing up in the US, I had never heard of "gas mark" until just now! Our ovens are usually marked in degrees Fahrenheit.)

As for how long, this tends to be an easy one for most cakes: once it starts to look done (it's starting to turn evenly brown on the outside) stick a toothpick in it. If the toothpick comes out clean and dry (perhaps a few dry crumbs on it) then the cake is done. If it's got batter or wet clumps, wait a few minutes and try again. (Follow the link for more ways to test!)