Making larger rich fruit cakes

Hi Lindsey,

I have successfully made a gluten free version of Delias Christmas cake, using gluten free plain flour. However, I need to make it for the bottom layer of an anniversary cake. The whole cake (2 layers) need to feed 70-80 people and therefore the bottom layer needs to be bigger than the 20cm specified in this recipe. Do you have any tips on making this cake bigger? Including baking times? Would really appreciate your advice, or any alternatives for a fruit cake.
Many thanks
Rosie