Very good vietmanese food, easily walkable (5 mins) from Old Street Tube and just south of the famous Hoxton Square. The pho is delicious but my particular favourite is the mackerel baked in banana leaves. Service is something to be desired, and you want to do your best to avoid being get in the crowded basement if you want to get attention.
The only better place I can think of for Vietmanese is Song Que on Kingsland Road, but while the food is better, the environment is far worst and the waiting staff regularly bad tempered. It's also harder to get to.
You can't go wrong with Cay Tre. Enjoy