Thanks GreGorGy and WAFarrah,
I tried LittleSnitch and ran it for 3 hours - the 2 services that were running constantly pinging external sites were vmnet-natd and mDNSREsponder.
These services are related to my VMWare fusion installation, strange how these run even when I am not using a VM - but I'm still not sure whether it can be these 2 services that can be consuming so much bandwidth.
I have also installed SurplusMeter which shows me very nicely how much bandwidth is being consumed, but unfortunately still does not tell me what is consuming it.
Between 23:59 last night and 07:00 this morning 400MB was consumed - the only thing I had running was iChat with a gtalk connection in it. No browsers were open, I had taken my mail offline, stopped MobileMe syncing - I do have Xmarks for Safari installed - it is possible that this might be the culprit, wouldn't think that I had 400MB worth of bookmarks though.