Core Lock Services recently converted a passage lever set into a Gainsborough Trilock for a customer in St Ives. The Gainsborough Trilock can be left in 3 modes:
Passage mode for access from both sides at all time
Locked mode where the outside requires a key to unlock the lock but the inside can always be opened
Deadlocked mode for ultimate security. This locks both sides of the lock and requires a key to unlock. The latch is also 'thrown' further into the striker plate and door frame.