The first thing I would do is speak to PALS at the hospital - be polite but firm as their parking regime is clearly not meant to punish genuine visitors, much less those who have actually paid.
If this were to go further, including to court, one issue you may face is that it would essentially be your word against theirs, although with witness statements from both of you, and if the kiosk attendant does not provide a statement to the contrary (a failure you'd be able to draw the court's attention to) then the balance of probabilities could swing in your favour. It would be a risk though.
PALS is by far the best route to try first.
