Simon,

on the left hand of your saved HTML representation, you may navigate from one sheet to another using the sheet list. The name of the Sub-Process sheet always equals to the name displayed on the corresponding Collapsed Sub-Process. So navigation is unambiguously from Collapsed Sub-Process to its content. The unambiguous way back is done by the browsers 'Back' button.

(But I agree, hyper link navigation would feel more straight forward. I will store and forward your request to engineering for a future version of Process Modeler.)

Manfred