Apologies I wasn't being clear.
Yes I agree it's a promotion into a vacancy that became available.
But in every place I've worked there are two types of "promotion" - one being a move into another role having applied for a vacancy (a person could also move into a vacancy that was a sideways move ergo not a promotion!) and the second being where people could be "promoted within role" eg if it was deemed they were operating at a higher level within their current role.
There is usually a process by which their grading could be increased. It's often referred to as "progressional" promotion (and is not the same as a simple pay rise).
The issue is not that OP was "overlooked for promotion" but one of whether she was given the same opportunity to apply for a vacancy.
Where I work, employees can still access our vacancies board while on maternity and HR can send out emails with current vacancies to an email address that's been agreed with the employee prior to going on ML. Their line managers also do keep in touch days during which they are expected to keep the employee updated with any significant developments in the team and the company.
If the OPs employer has not been keeping her informed then absolutely they are in the wrong!
The OP said herself though that even if they had, she probably wouldn't have applied as the vacancy was advertised as temporary and needed to be filled sooner than her ML was due to end.
Her manager has offered an alternative to help her progress and OP was initially happy with that.
The issue now seems to be that the position has been made permanent. But I can't see how her employer has done anything wrong in making a temporary role permanent. As I said, they may not have known at the time of advertising that it needed to be permanent. This is perfectly normal.
Now if the OP was saying that her employer advertised the role as temporary in order to knowingly exclude her due to maternity, all the while knowing in the future it would be made permanent, then she'd have a point.
Not sure how you could prove that at a tribunal though.