Hi Simon,
The 'magic' is hiding in plain sight in the detail_in form. If you follow the logic around, clicking the 'create problem' button sets a flag 'parent.createProblem' and does a Save. One of the first things the detail_in form does is to look at the 'parent.createProblem' flag: if set, the form calls 'create_new' with a preset 'type=P' and a reference to 'frm002', which is a form within the detail form. Find '<form name="frm002">' and add the 'SET.' element of your field as a new hidden field to the form, modelling it on 'impact'. For example:
Publish the form and test.
Hope that helps,
Regards,
James