If I want someone to pay me on a fortnightly basis for unlimited attendance, how should I do it?

There are a couple of settings combinations you can use in your Pricing Packages (Settings > Pricing)

You need to decide first if you wish to create this Pass to be one-off or recurring (automatically billed to the customer until cancellation).

If Max Classes per Period is left blank it means unlimited. You can also specify a number eg. 10, 20, 30.