Thanks - just had a look at the Oyster▸ card. They only want a ^3 refundable deposit, so I'll probably go for this.
For only ^3 the Oyster card is worth having. Appart from being slightly cheaper for many tickets, its also a lot easier to use (just remember to touch in and out even when barriers are abscent or open).
Buy one from the tube ticket office at Victoria. It will only take them half a minute to issue it. IIUIC, you can "load" your rail card onto the Osyter card so you get RC discounts automatically when applicable. (ask when you buy it).