OK, so the question we need to ask now is—Where are those pages currently located on your server? They should be inside the /jonathanbarton1/ folder, but they aren't there. (Are you sure you have uploaded them at all? Check in your FTP program to make sure they actually got uploaded.)
(Note also that in your online version of the menu, you have a gap before the gallery URL:
<a href=" gallery.html">
Remove that gap before the g or you will have a problem with that link.)
Edit:
I just did some fishing, and discovered that the reason your links are not working is that your file names start with a capital letter (e.g. About.html) while your links point to files without a capital at the start (e.g. about.html). As a rule, never use capital letters in HTML or CSS code. It's just one more source of trouble you can do without.

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