Three of us London birders did the Oystercatcher bird race last May, winning with 90 species between the hours of 6 am and 6 pm. We did this by starting at Hyde Park/Kensington Gardens, then doing Rainham Marshes (west end via Rainham station) and finishing up at Walthamstow Reservoirs to mop up on things like Lesser Whitethroat. a few species were fortunately spotted from the train between Fenchurch Street and Rainham, so keep looking! And we still didn't get Green Woodpecker, despite our best efforts (there will always be a 'bogey bird' on these races, I guess) ...
100 could well be doable with more time and a more ancient woodland site (say Epping or Broxbourne, or Belhus Woods near Rainham), but there probably won't be anything at Crayford that wouldn't be easier at Rainham ...
Good luck!
D