Active main menu link I need to set an item in the main menu as active even though I'm using separate sub menu blocks. What is the best way to do this? Due to particular styling I cannot just put them under the Category headings (like breadcrumbs).

The problem I came across is that the main menu item isn't active, as it doesn't know that the user is in the sub category of it as there is no link back. I can't find a way to set a particular main menu link as class active.
You should look into menu position. You can set active menu trails based on path or node type and they have a simple api for extending the functionality.