We call it a convection oven, because it sounds like we’ve gotten so much more for our money. You have to admit, fan oven sounds a bit more pedestrian.
Anyway, one of the great benefits of a fan oven is that you can stuff it almost completely full and everything is still supposed to cook evenly because the fan does such a great job of circulating the heated air throughout the oven cavity. Assuming both of your cakes are in the same-sized container, they will be done at the same time. Theoretically, all that business about turning cookie sheets or pans around or swapping cakes or pies between the top and bottom racks halfway through cooking is unnecessary with a fan oven. (We are not convinced that all fan ovens are equally good, but that’s another story.)
Because fan ovens do a better job of circulating the heated air, they tend to cook foods more quickly – sometimes too quickly. The convection setting on one of our ovens is called “speed cooking.” Hence there is a general admonition to reduce the cooking temperature by 25°F (12°C) when you use a fan oven for a conventional recipe. As you become more familiar with your particular oven, you will learn what adjustments in time and temperature work best for you and the foods you cook. For anyone just beginning with a convection oven, you should be vigilant. Many dishes will be done sooner than you expect.
While we’re translating your question for Yanks, very few of us know that a hob is a burner. Be-Ro is a brand of flour (in the case of this recipe, self-raising) produced by Rank Hovis, and its Easy Fruit Cake recipe is online for all of us to enjoy (click on the link, then click on Cakes, then click on Easy Fruit Cake).