Well, it needs cutting when its actively growing, though shouldn't be cut too short at this time of year, set the mower on high.

I was told by the supplier not to mow my new turf for three weeks and water very well.

I have also just layed new turf. Best to leave it settle 3 - 4 weeks and avoid walking on it either. Water regularly and cut with mover on high setting for the first cut.