I have a kings ransom in cake tins, but I'm stuck for which would be best for a Christmas cake. Should I be going for one that's quite deep? Or will a fairly normal 7cm depth one be fine? This recipe I plan on using is the one from your book 'Nigella Christmas' (which I love!)
The most common size of Christmas cake would be a 20cm/8 inch diameter cake, though there is the handy chart with ingredients and times for larger and smaller cakes. However you do need a fairly deep tin for all sizes of Christmas cake. A regular sandwich tin (cake pan) will not be deep enough - you ideally need a loose-bottomed cake tin or springform pan which is around 8cm/3 1/2 inches deep.
A 7cm deep tin should be fine as the cake does not rise dramatically, but do be sure to line the tin with a double layer of baking parchment (parchment paper), with the lining coming above the lip of the tin, and if the batter is quite close to the top of the tin then we would recommend wrapping a double layer of brown paper around the outside of the tin too, to reduce the risk of the top edges of the cake becoming scorched. Some people also like to make a small indent in the surface of the batter, in the centre of the cake before it goes in to the oven so that you get a nice flat surface for decorating.