Rough figures but according to orange 150/1/2/4 I used 0.49mb to 5pm 26/3 - I was only in my wifi area, with 3G on, using Deezer only during the time I was doing my experiment (e.g. having 3G switched on - it's been off for a week and a half til I get to the bottom of this data hungry network). Deezer was crazy slow and downloaded 6 songs in the space of hours. I didn't wander with the phone. Technically I should have used 0 data.
According to the Call Credit app and Orange online I have used a total of 4.89mb (earlier today) and I've just checked again on both and it's still 4.89mb.
This is the interesting bit. Today at 8am I swtiched my internet on for 6 minutes when I was in the 1 room I can't get a strong enough wifi signal - quick browse of WPCentral, FB and quick look in the store. No streaming, no you tube, no downloading. Switched 3G off after 6 minutes and it's been off the rest of the day. 4.4mb in 6 MINUTES? 25mb is meant to last you 9 hours of surfing/150 texts with attachments (if I remember correctly what the guy told me last week justifying the ?1 a day for internet once you go over your limit - even if you are only on it to get into deezer :/ ) Therefore you "should" be able to surf for 1 hour and only use around 2.7mb data.
As 'my orange' on pc and the app have updated I tried calling 150, they only have data usage til 5pm yesterday 0.49mb She was quick to tell me to wait til bill is generated then query. Ermm NO! Tried that last month only to be told that once the bill is generated they can't tell when you used your data. Think I'll just keep phoning daily til they get to the bottom of this issue. Only way I can combat it is with logging daily what I am doing with my phone Wifi only/Deezer/when 3G is on and what I am using phone for at that time. Tedious but I want answers.