I do think this is a gastropod shell but that's not much of an answer. All snail-like shells are gastropods, and I'm not sure of the age or type of gastropod. So I will get back to you when I have any more information.
Sorry for the confusion - I must have been in a rush! Btw, I've been changing titles recently to make the forum easier to browse and search and to help me keep track of enquiries that have and haven't been answered.