How to debug excessive iPhone data usage?

I'm in the UK, and recently changed from a pre-pay carrier (GiffGaff) to EE. My data usage with GiffGaff was about 100-125 MB every two weeks; with EE, I've used nearly 250 MB in the first 5 days. My usage is the same: email, web, Twitter, and some Maps. (Both with my last two weeks with GiffGaff, and these past few days, I've used Maps on a driving trip.)

So I'm trying to figure out what's going on. I know I'm not using that much data, but something on the phone is using data. This is clearly a carrier issue; either the carrier settings or something that has changed. I know you can access Console logs via Xcode, but I don't see anything helpful there.

Does anyone know how I can debug this? I'm ready to go to EE and cancel my contract, because at this rate, the data will cost me a fortune.

Some networks measure data use in different ways to others to get more profit out of it so maybe consider changing to a network that measures data as less?