Cache Plugin Pro Version – Mobile Layout Issue

Hello Graham,

Thank you for reaching out.
Can you please disable the settings one by one in Performance>general settings, save the settings and purge the cache after each setting is disabled and see which one might be causing the issue?

In Performance>Cache Groups>Manage User Agent Groups, you can Create (or simply enable the existing) a group of user agents by specifying names in the user agents field. Assign a set of user agents to use a specific theme, redirect them to another domain, or if an existing mobile plugin is active, create user agent groups to ensure that a unique cache is created for each user agent group. Drag and drop groups into order (if needed) to determine their priority (top -> down).
If you are not using any AMP, the only thing that you need to do is Enable both Groups (High and low), and select the theme that you are using in the dropdown menu to ensure that a unique cache is created for each user agent group. Drag and drop groups into order (if needed) to determine their priority (top -> down).

The two default User-Agent Groups are covering all notable groups. If you wish to add any other you can do so by Specifying the user-agents for these groups. Remember to escape special characters like spaces, dots, or dashes with a backslash. Regular expressions are also supported.
Make sure to save all settings and purge the caches once the groups are enabled.
Please make sure to check the screenshot below and let me know if this helps.

Thanks!