Cheers NickB - that does help a lot.

I have one more question though. I always start my journey from a National Rail station that doesn't use Oyster (i.e. there aren't any machines) and I then change at Stratford onto the tube. Can I still use an Oyster card (and wave it to any ticket inspectors who do turn up from time to time) or do I need to stick with a paper ticket for now?