I often get called to this predicament...lost key or broken lock..
There are various tools readily available to pick mortice locks..but they are expensive, and as you only want to open the door once...they are out of the question.
Depending on what quality of lock you have will determine how you open it....
If it is the cheaper 3 lever lock..as a rule I get a hacksaw blade and cut through the latch...just lower the blade down between the door and the frame until you feel the latch, and get sawing...it's normally brass and you will be through it in no time.
If its a better quality 5 lever lock it may have an anti- saw hardened steel roller fitted in the latch...(you cant see this unless you have the door open)...once you start to saw you will find the blade seems to skid without cutting...this means you have hit the hardened steel roller...
Not a problem....get a grinder with a thin metal cutting disk.
You need a slightly larger blade than the common 115mm, to allow you to get a deep enough cut.
I've done dozens of them...and the grinder never fails.