Hi Fran
Welcome to the site.
Ooh well done you on getting the job of making the wedding cake!
The first thing you need to do is work out what size tiers you are going to use for the wedding cake and get/buy the appropriate tins so you can work out sizes and depths which will help with your practice sessions. I would suggest using the same brand of tin so that the wedding cake has the same depth to each tier.
The only way to figure out if your recipe works at other quantities is to try and apportion it appropriately, i.e. half the quantities for say a smaller tier 4 inch tier or 1.5 times the quantity of your recipe for say a 6 inch tier, the other solution is to make your normal quantity or double it up and then distribute it between the tins you are using. That way you will know how much you will need to use to fill each tier to the required depth you need.
It sounds to me you have a very quick baking fruit cake, however, depending on the depth of your tin will depend on how long it takes to bake. Deep tins (that are about 3 inches in depth) will take longer to bake and tend to need to be baked on a lower heat. Otherwise you will bake the outside but the inside will still be gooey.
If you were making a 4 inch tier I would probably check after 25 mins to see if the cake is done (seeing as you said it only takes you about an hour to cook an 8 inch) test it with a skewer in the middle if it's comes out clean it's done if not place the cake back in the oven and bake for further 5 minute intervals and then check it again, continue to do this until it is baked.
Again for the 6 inch bake for approx 45 minutes but you could check after 35 mins to see if the cake is baked but only open the door if the cake looks baked, if the cake looks no where near baked leave it for a further ten minutes, then do the cake test with the skewer. The best thing to do with the cakes is use your own judgement and have a pen and paper by the oven so that you can jot down the times you have had them in for. Once you have worked out how long to bake them for bake them again on another day and do them for the full duration and then see if you are happy with the results. Just remember you can bake cakes for longer on a lower heat and if you wish to add more moisture to your cakes whilst they are baking then add a
water tray to the base of your oven. This absorbs into the cake and makes it a little more moist.
Tip:
The deeper the tin the lower the temperature and the longer the cake takes to bake.
For an
even rise and bake surround your cake tin with baking wrap whilst baking.
If the top of your cake is done and the middle is still uncooked place a butter sheet over the top of the cake to prevent if from browning further.