Is pay-per-use Oystercard cheaper than... an annual travelcard?

Yes. For me, it's over £100 cheaper to just 'pay as you go' with an
Oyster pre-pay card.

Good grief.

I've done a little work to help you calculate your own savings by going
Oyster and ditching the travelcard altogether... try:
http://www.mediauk.com/content/oyste...calculator.muk

Naturally, it's not always going to be entirely accurate: but I'd be
interested in your thoughts.

James