A substantial portion of the Baronial army at Lewes was provided by the London Militia.
I based the flag in this source: it dates from the time of King John*. The flag of London, since sometime in the 14th century and before Wat Tyler's rebellion, has been what looks like the cross of St George but with a sword in one corner (as shown here). This is the flag of St Paul**. I really couldn't decide what kind of flag to use. I originally painted it as a banner, which is perhaps more suitable for the description of Fitzwalter carrying it on horseback. I then saw a modern version which is a vexillium type, still used by churches. Since the flag was donated by the cathedral, I thought I'd do this kind. Unfortunately, I prefer the painting I did on the other banner! I think I shall have a third attempt, with this seal as a guide.
** Worcester Cathedral is also St Paul's - I didn't know until today that they use the same arms as London.