Help, locked inside by Night Latch Deadlocking itself!!

Hi all,
Fitted a yale night latch about a year ago, no problems works fine. Couple of weeks ago we came down to go to work and couldn't get out!!.

Had to go out round the back round to the front door and turn the key in the lock all the way round again. It's as if the lock deadlocked itself. Paranoid that someone got a copy of the key, I ordered a replacement barrel and fitted it the next day. That was a week ago, but the same thing happened this morning. I know nobody has the keys except us now.

Latch seems to work fine once it's unlocked, the barrel doesn't move when operating the latch from the inside.

Has anyone seen this before? or Can anyone offer a possible explanation?
At the minute all I can think of is:
* Either me or my partner is sleepwalking going out and deadlocking the door from the outside then going back to bed (unlikely)
* Someone has got a copy of the key (unlikely)
* Somebody is having a laugh and trying to turn the latch at night? unlikely, don't see how if they don't have a key.
* Is there something wrong in the body of the night latch? possible, though it seems to work fine.

Please help if you can, it's a bit unnerving not knowing if the next time you try to get out whether your own door is trying to keep you hostage

Cheers

It's possible that the cylinder connecting bar is too long on the cylinder, or a bit rough on the end and getting jammed up in the mech. Just one possible cause

are you shure when you come in last thing at night your not turning the the key to the double lock position when removing the key
and slamming the door so the keep on the frame jumps the bolt!!!

Cheers chippyphil, will have a look at the bar length. I cut it so that I had a good length in the latch. Didn't seem to stick though the length could be plausible.

I definitely didn't turn it all the way round big_all, It would take longer than normal so I'm sure I would have noticed.

Thanks for your advce guys.