As you say you've just had it fitted ask the plumber to come back and adjust the toilet cistern and show you how it's done for future reference. All cisterns have an adjustment you will have to look around it or if you can't get them back [don't recomend them] call in to a local plumbing shop with a digi picture of the inside of the cistern they should be able to tell you how to do it.
This is the float and arm type for adjustment.
At the water valve inlet end you will see a threaded rod with a lock nut and the end of this bar as the water rises the arm rises and the bar presses against the diaphram plunger eventually shuting the water off until you pull the flush again. slacken the lock nut and adjust threaded bar by turning it against the plunger a few turns then let the cistern re-fill see what the level is, adjust further if needed.
