[REF] Use form property rather than formValues to pass generated invoice ID

[REF] Use form property rather than formValues to pass generated invoice ID

formValues is 'sort of' the array for 'submittedValues' but it has stuff added to it
& passed around. This is part of an attempt to make it clearer where
various values come from to faciliate cleaning up the code and
removing some of the very many arrays in use.