Dear Abouttoblow, I've just read this thread and am sending high fives your way! I've worked with piss takers too and felt the anger you and your colleagues do.
For what it's worth, here are my thoughts.
Your colleague is just very self-centred. She doesn't think to use the flexi system for personal errands and assumes you all do the same. She was astounded that you lot make doctor's appointments on your days off or make up the time when you have to leave early/come in late. She works around the convenience of her own life and it doesn't cross her mind that anyone else thinks differently.
She has a very low ethical code. She can't understand she's stealing, she has no sense of fair play or justice, she tells lies (wedding ring..??).
When she's called out on her behaviour as you SO BRILLIANTLY managed to do by getting Personnel to hammer home the rules, she takes it personally and feels entitled and special and victimised.
In short, she is likely some sort of narcissist. You can't reason with people like this. Keep away, smile internally, you have made your point and effected a change.
I still think you were right to do something about it, all the "keep out of it" types (who have all gone silent now that you have been proved to be rational, correct and achieved a positive result) are pretty feeble; they're the ones who allow bullies to thrive by their silence, or fascists or extremists to get into power through their apathy.
Good luck to you, from what I can tell, you are working in a vital sector and we need people like you who actually give a sshhit.
Oh, and Madam "Personnel doesn't exist anymore" and "HR is a business strategic partner"? Puhlease. You hire, pay and dismiss people. Get over yourself.