Hi and Welcome to the forum.
Some apps do update in the background which does use data but not in the quantity you are going through.
I would suggest you call o2 on 4445 to ask them for a help to see where the data has been used and that your new tariff is set ok and you are properly on Pay as you Go go go. If any credit has been taken in error then they will return it usually.
Check in your Myo2 to see what tariff it states your on.
When you change tariffs it would not kick into effect until your next anniversary date however you can ask customer services to override that for you.
I'm also on Payg go go go 20 and get 1gb data.
Now if I go over this I will too incur the £1 per day data charge. This is not something new that o2 charges.
http://www.o2.co.uk/tariffs/mobileinternetdata-unl-int-popupHowever there has been an update to some t&c that took effect from 17/9/13 but thats for Web Daily.
http://www.o2.co.uk/termsandconditions/mobile/pay-and-go-web-daily-web-daily-bolt-on-monthy-web-bolt...Only Pay Monthly Customers are not charged for going over their Data allowance but their data speeds are throttled.
A good app called Onavo for yiur phone ,which you can download will collate what's being used data wise on your phone.
http://www.onavo.com/apps/android_countYour can choose to purchase a Data bolt on through your Myo2 to keep on using Data with a decent allowance but take into account this too only lasts a month , does not roll over and if your Payg anniversary date kicks in to refresh all your normal allowances the bolt on is lost.
Ref Network Service.
Double check your Service Status here to see how your o2 masts are.
If they are showing ok and your confident their is something wrong then you can challenge the results with o2
http://status.o2.co.uk/O2 has a service called Tugo which can be used on a persons home Internet wifi but at this time that's only for Contract Pay Monthly Customers.
Try ;
Put your sim in another phone so test that your primary phone is not at fault.
Remove and clean your SIM card.
Replace the SIM card by visiting an o2 shop where they will transfer your tariff and number across. Takes from approx 2hrs to around 24 to fully reset. Turning your phone off and in throughout this period helps reset the service.
Try switching to 2G.
Look in your handset settings for an option to reset your network settings.
O2 are working on their network to upgrade it to 4G and as such the 2G and 3G service will improve too. This by 2015.
http://www.o2.co.uk/network/futureI know how frustrating it is as in my area the masts have been up and down for the last 4 months too !
😞Regarding Text messaging.
You only have to be connected to Data to send and receive MMS messages. It does not actually use data but has to have connection. If your sending texts over 160 characters and pictures the normal text typically converts to a MMS.
http://en.m.wikipedia.org/wiki/Multimedia_Messaging_ServiceFinally you do have the option to leave o2 easily as your on payg and find a network that offers a stable signal where you live , commute and work etc. remember to ask for your PAC code. And you can unlock your handset for £15 (must have been 12 months as a payg customer.)