This deliously nutty and rich traybake is from the recipe on page 188 of Mary Berry's Baking Bible.
The whole cake has only 40g butter which seemed to be on the low side given that there is 350g flour, and 300g of almonds and walnuts. I guess this is a nutritious bake. I cut the cake into two and fronze half uniced. I cut the iced cake into 12 pieces and stored that in a tin. It keeps well, and was even better after the first day.
We both felt that the lemon icing did not add that much. When the second half was brought out of the freezer a couple of weeks later, I made a small amount of butter icing, using maple syrup instead of the water, and this really went well with the cake.