The biggest difference is that this year's cake was undercooked, despite passing the skewer test and being cooked for a bit longer than specified. It's still tasty, but definitely squidgy in the middle, and despite covering for the last part of cooking I would say the top is a bit burnt. The recipe is also wetter overall: it has less fruit (over 25% less) and less flour, but the same amount of egg and alcohol plus the juice of a lemon and an orange.
Other errors were mine. I fed with brandy, which is just not as tasty (to me) as sherry or rum, and I made the icing wrong, omitting one egg white, so the icing is very crumbly. (However, a useful note - the egg whites make this a good partner to the Christmas shortbread recipe which needs yolks.)
I don't roll out marzipan for the whole cake at once, but match up a circle (drawn around the outside of the cake tin) and a couple of strips wrapped around the sides. The icing recipe given by Mary (3 whites, 675g icing sugar) is quite a lot for a 20cm cake, and personally I prefer a thinner layer of icing. However it keeps well and is good for piping on biscuits. I piped filligree stars on the top of the cake after lightly pressing a biscuit cutter into the icing to give a template.
Next year I also need to make multiple smaller cakes, as a 20cm one is too much for us really. I see that various blogs and recipe sites suggest using small baked bean tins, which is genius. Must remember that it would use more marzipan and icing, to do them all over, or much less, to just do the top.