I have added the code as suggested for the view full site and added it to the footer.
<a href="http://www.jmcsweeneyphotography.co.uk" onclick="Mobify.desktop(); return false;">
View Full Site
but it is not working and is just refreshing the page.
Is there anything I have missed?
Your full site link is actually set up correctly. The reason you are just seeing the page refresh and not unmobify is because you have the Mobify preview hash appended to your site URL. If you look at the URL in your preview window it probably looks something like this:
Everything that comes after your site URL (#mobify-path=true.. etc) is designed to force you to see the Mobifed version of the page. We use this method to preview a site that has not launched, so regular users will never see this. If you are viewing a page with this hash appended to the URL, the full site link won’t work.
To test out your full site link, try navigating away from the homepage, and then back to it. This should clear out that preview hash, so your full site link will work.
There’s a couple resources available that describe the process for both sending a user to the desktop site (opting-out of the Mobify adaptation) as well as providing a way to return to the adaptation:
Have a look through and give it a try – let me know if you run into problems.