bantamdave41 when i forget my oyster card i use my contactless bank card, but remember to scan in and out always. Alternatively you could get a one day travelcard zones 1-4 which was about 9 quid the last time i did that, if you go with the travelcard option get your ticket in advance as a lot of time will be wasted queueing to buy one otherwise.
Have a good trip and stuff them stupid!!
Just a quick one for all you 16-25 aged fans and the 'Seniors! If you have a 16-25 or a Seniors railcard, then you can get an Oyster card configured to give you a third off your travel in London. They can update existing Oyster cards. It takes about 10 minutes but will be valid until you no longer have the 16-25/seniors card.