Mix contains 500g sr flour, 500gr sugar, 500gr butter& I've wanged it all in a large oblong tin. I've increased a v old BeRo recipe for a cake made with one fifth the amount of ingredients & that tells me to cook that amount in one tin for 40-45 mins at 160 in a fan oven.
My q is how long(ish) will it take to cook & should I lower the temp to avoid a black crust & raw in the middle disaster.
Probably a bit late now, but you need to cover the top to stop it drying out and burning. You would also be advised to use a cake cone or dariole to get some heat in to the middle of the cake to keep the cooking time as short as possible (to reduce the risk of over cooking the outside).