For the Simnel Cake can I use self raising flour instead of plain? And can I use lemon curd instead of apricot jam?
Nigella's Simnel Cake (from FEAST) is a light fruit cake that is often eaten during the Easter period. The cake is not as heavy as a regular fruit cake but not as light as a sponge cake and is made with a mixture of plain (all-purpose) flour plus a relatively small amount of baking powder. Unfortunately the amount of baking powder in self-raising flour doesn't really make it suitable for this cake, even if you omit the baking powder, and it will affect the texture of the cake, so we would not recommend it as an alternative.
The apricot jam is used to glaze the cake before the marzipan is added as it helps the marzipan to stick to the cake. Lemon curd will not be quite as "sticky", but you could use it as an alternative.