Mobile Custom Pages
I thought I'd post this here as it had me stumped for ages before I stumbled across the solution (almost by accident).
To create a custom page for mobile:
In Studio select the Page tab and click New
For the page name, use the following convention:
[MobilePageName].[AnythingYouWant]
For example:
e.g. MobileCategoryPage.MyMobCat
For Page Layouts choose Mobile Page (unless you want a different layout).
Optionally, for Copy from page, choose the mobile page you are overriding (this will just pre-populate the page with the same components).
Click Create
Set the page components and click save.
In admin, pick the page you want to set the custom layout for from the 'Settings for' menu (just below the main title).
Go to Content, and select Custom Pages. If all has gone well, there will be a drop down list for the Mobile Page, allowing you to select the custom page you just created.
You'll need to repeat the last two steps on live after you have release the changes to production.