http://www.daysoutguide.co.uk/oyster
This may be of interest for those of you who will be in London within the next month wishing to travel during off peak hours.
It's pretty simple. I printed a voucher today. All you need is an Oyster card number, your name, dob, email, a post code (doesn't have to be yours) and the date of travel. You select the destination and a voucher will appear.
The travel must originate from the London station indicated but you can join the train at any point along the route.
Happy travels.