Hello, I've got a cake to do which has a little tent on the top of it with some hiking boots and a rucksack and I've started making the tent this afternoon and its starting to get really heavy and am wondering if I need to do anything special when I put it on the cake? Would it need some kind of support or will it be ok if the fondant has been left to dry overnight?

If your topper is getting heavy enough for you to worry aqbout it, it would be a good idea to dowel and support your cake as if it was to have an extra tier up on top.
Place dowels or straws or whatever your choice of support mechanism is, into the top-most tier of your cake (which, if anything other than a one-tier cake, should have supoports for that tier as well), and place a cake board at the top of your finished cake. Decorate the cake circle and your topper however you wish, placing over the straws/dowels. HTH!

If your topper is getting heavy enough for you to worry aqbout it, it would be a good idea to dowel and support your cake as if it was to have an extra tier up on top.
Place dowels or straws or whatever your choice of support mechanism is, into the top-most tier of your cake (which, if anything other than a one-tier cake, should have supoports for that tier as well), and place a cake board at the top of your finished cake. Decorate the cake circle and your topper however you wish, placing over the straws/dowels. HTH!