BSZ-Blake

Not quite the exact same issue you have as mine wouldn't unlock, rather than failing to lock, but otherwise sounds very similar to an issue I had. I just completely replaced it. It seems like a pretty sealed unit so I'm not sure how much luck you're likely to have trying to repair it. Ended up taking mine to a garage to get it replaced as it's tucked behind the inner door skin which if I remember correctly is held on by a combination of bolts and rivets. I'm sure it's probably possible to replace yourself, I just didn't fancy drilling out rivets with no idea what else might be behind there