I know you can buy 1gb on the PAYG sims. Is there a similar system for those on monthly contracts? Thanks
Not AFAIK, you will get hit by a £1per day charge no matter how little or much you use till next bill refresh data.
You would need totally turn of DATA and phone will not do certain things without it even lesser background data disabled gives you pops up asking for it to be enabled.
This happens even on WIFI as you can see your phone uses a little bit of Data even on WIFI for some Apps.