A little update for those who may have experienced the same problem>
Forgive me for not remembering all the technical details
After talking with Vodafone (with a clued up advisor this time), it turns out that even though I switched off the network to browse via my WiFi connection, when I switch the mobile network back on, it connects via GPRS (I think) to 'update' something or other, and charges for the data usage. Over the course of a month this adds up...
Vodafone were pretty good about it and waived the charges for this month, giving me a bill reduced from £37 to £7 with next month's bill at £34. As it should be around £20/month, it works out ok.
They've now disabled mobile internet on my phone which is fine by me as I only bother with it when I'm at home, using my own WiFi connection which should now be free (Technically, it always was but you know what I mean).
Hope this helps someone.