I haven't been able to solve the problem yet but I think I'm getting closer. I've noticed that the data drain stops when my iMac is turned off, and that the data drain on the iPhone starts immediately once the iMac is turned on!!
I turned off my iMac on Friday and had no data drain all weekend. As soon as I turned the iMac on today the iPhone "Documents & Sync" started to go crazy again. I logged out of iCloud on the iMac and the drain on the iPhone stopped! So, I don't think this is an iPhone hardware problem at all!
To me it seems it's a bug in the iCloud service. Although all the configurations on the iPhone are saying that it should not use 3G, and that it should use the WiFi, it still uses the 3G somehow for some sort of synchronisation with my iMac. This is clearly a bug as I don't have anything on my iCloud drive and I have set Calendar/Contacts and Notes to sync only on WiFi. Apple please sort this one out. For now I'm switching cellular data off on the phone when I'm working on the iMac. It's the only solution at the moment.