Just making mine, fruit soaking in orange juice and brandy, will make later today. As with any cake, if it looks done, if you press the top and it springs back, then it is most likely done. I do put a skewer in, and if it looks as though it just has marzipan instead of cake mix and marzipan sticking to it, then I am happy to take it out. It is very easy to overcook and end up with a dry one.
Prefer to make my own marzipan too, no artificial colouring and only the natural almond flavour.
A Simnel cake needs to have a topping of marzipan and the eleven balls of marzipan on the top, to represent the apostles minus Judas Iscariot. But of course it needs nests, mini eggs, fluffy chickens, fresh or sugared primroses, anything else you can cram on to represent new life!