I would definitely call to complain again, especially if they've not given you the credit.
You can download an app called Network Traffic Stats lite
, that should tell you exactly what's eating into your data allowance and making you go over it.
It won't be an unlimited plan you have though unfortunately, the most Orange do is 1GB as far as I'm aware so it'd be good to find out exactly what you've got. My girlfriend has the Orange San Francisco which came with "free" internet but it was a meagre 100MB so they can be quite crafty with their wording.