Mix the tuna, sliced olives and chopped tomatoes in a bowl.
Fill the cannelloni tubes with the tuna sauce. I rest the tube vertically on my hand or one of my fingers to cover the bottom so that the sauce doesn't go straight through it then fill with the help of a spoon (use the handle of another spoon to push the mixture in) then place on dish and start again.
Pour 250 ml water (from the cold tap) into the inner pot. Trivet in place. Dish with the cannelloni on top.
Make sure the sealing ring is in place properly by pressing it down following its circumference several times. Steam release handle pointing to sealing.
Press Manual / Pressure Cook and press the - and + buttons to programme 25 mins with 15 mins npr. Check it's cooked and if not add more time or just leave it be with the lid on as pasta keeps cooking in the residual heat.