my bought turf is dying in places

Do you have a dog? Dog urine can ruin grass... www.primrose-london.co.uk stocks a suppliment called dog rocks that you put in your dogs water bowl. They are harmless to the dog but stop its urine from scorching your grass.

The only other thing I can think of is that it's not getting enough water... though if it is just yellow and not dead I would say leave it so that the grass puts down longer roots - if you pamper it too much by watering it then it won't put much effort into growing its roots.

Is the yellow spread all over or is it largely in one area?

New turf needs watering every day morning and evening to get it settled in for about the first couple of weeks and especially at this time of year .

I assume you didn't notice any problem with the turves when you first unrolled them? And did you make sure there was good contact between the underside of the turves and the soil? Sometimes, if there's a pocket of air underneath (usually because the ground's not completely flat) the turf dies out in that area by going yellow first.
Inverglen is accurate in what he says about watering - it will take the turf some time to penetrate its roots into the topsoil underneath it and until it does, it needs to be kept thoroughly watered, I'd say with a sprinkler every 3 days for a minimum of half an hour on each area for up to the next six weeks.
You say the area was properly prepared, so I'm assuming you dug over,levelled, trod down, raked up and got a good, loose fine tilth which you smothered with Growmore or similar before laying the turf.

i totally agree with inverglen after laying the turf you need water water and more water i bought some from b& q checked the quality and just watered never letting it dry out and its lovely