HERE ARE THE BASICS:
Infinite Ammo Cheat
During the FMV that precedes the main menu, press UP, DOWN, LEFT, RIGHT, TRIANGLE, UP, DOWN, LEFT, RIGHT, SQUARE. Repeating the code disables it.
Full Recovery:
When you have been shot, and you badly need health walk up to a wall (any) and stand next to it close he will then lean up against it, it may take a few seconds but wait till he gets off, he will have a full recovery.
Kelvin Deamer mode:
Successfully complete the game to unlock free roaming mode. Then, go to the golf cart that you will find in one of the parks. Press Up, Down, Triangle, Down, Triangle, Left, Right, Circle, to be transformed into a character known as Kelvin Deamer.
Skip intermission sequences:
Press R3 to skip some intermission sequences. Note: Not all sequences can be skipped.
Free Roam mode:
Successfully complete all standard missions to unlock the "Free Roam" option.
Double health:
Press Up(2), Left(2), Right(2), Circle(2), Down at the title screen. The sound of a woman groaning will confirm correct code entry.
Armored car weapon:
Press Up, Down, Left, Right, Square, Triangle, Circle at the title screen. The sound of a woman groaning will confirm correct code entry. Note: The weapon can only be used during missions.
Turn signals:
In free roam mode, for added realism you can use your indicators (turn signals). Press L1 to indicate right and L2 to indicate left.
Gas tank kills:
Shoot a gas tank when an enemy is nearby to easily kill them.
Health recovery:
If you have been getting damaged badly by enemies and are close to dying, walk up next to a wall. Your character will lean up against it in order to recover. With each breath your character takes, part of his bullet holes disappear until he is fully recovered. This may take a minute -- make sure that no enemies are attacking you.
Hidden vehicles:
You can find some hidden vehicles in the parks of London, such as a sooty van (Little Suzuki van with grass in the back) and a very fun golf cart parked behind huts and bushes around the parks.
Mark Hammond Mother
Press L2 L1 L2 L3 and you should hear a woman scream then your character will automatically change.