This weekend is Car Free Day in London — and part of this excellent initiative means free bike hire across the whole of London.
On Sunday 22 September (from from 00:00 to 23:59), all Santander Cycles in city will be free to hire.
All you need to do is download the Santander Cycles app and enter the code: CarFreeDay2019
Then you can explore the city by pedal power (consider using this tube map for cyclists).
Extra ride charges still apply and it's one use per person.