Cheapest price ever on Amazon and a good price for a lock that is certified to BS 3621:2007 standard. There are cheaper on eBay and alike but go for the Kite Mark version every time!
"Yale's maximum security BS1 night latches are British Standard approved to ensure the highest level of security for timber external doors.
They are Kite marked to BS 3621:2007 and are recommended by insurance companies. These security night latches have been approved by the Police Secured by Design initiative. The Yale lock is key operated from the outside and both key and lever handle operation from the inside with the internal handle being lockable for increased security. The latch automatically deadlocks on closing the door and the snob function enables the latch to be held back.
The lock uses a 6 pin high performance cylinder and has a 20mm bolt which is anti-carding. The BS1 night latches have a 60mm back set, this is the measurement from the edge of the door to the centre of the keyhole. Locks with a 40mm back set are normally used where there is restricted space, such as on a glass panelled door Case Width: 90mm Case Height: 90mm Case Depth: 33mm."