Tonight, while I was connected to my wireless network and the phone wasn't being used, it apparently connected to 3G and uploaded 60mb of info. That's now 140mb of data over the last five days. My data usage monitors have recognised that it's being uploaded, but not which app or process is using it. I can't image what would want to upload that much info and what it would be.
Strangely, none of it appears on my Vodafone account, but I have emailed them to ask for my data usage. Very odd behaviour.
I have been waiting for the 2.3.4 update before loading a new ROM, but I might not wait and go for a factory reset. Apart from this mysterious uploading (which uses loads of battery), battery life is now at a perfectly acceptable level. Very frustrating. I reckon it might me an app thing, rather than a SGS2 thing, judging by how I've not seen anyone else complain.