Responsive Menu that has a LOT of menu items

We have a responsive menu that on mobile just shows the top-level menu items (x8), but on desktop also shows hover dropdowns with service areas within the parent menu item.

The problem is we need the ability to show another level of navigation with the actual services within that area.

Here is the link: http://www.irwinmitchell.com

An example scenario would be:

Personal Legal Services
  >  Administrative & Public Law
       -  Disability Law
       -  Education Litigation
       -  Prison Law
       -  Professional Regulation and Discipline
       -  Social & Healthcare Law

So a structure like that needs to be shown for all the service areas within the main navigation.

Has anyone been presented with this problem before, or have any ideas what the most usable way would be to present further sub category's into the navigation?

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