It sounds as if it's about ready for its first trim. Try gently tugging a handful of grass to see if the turf lifts at all - if it does, then it's not firmly rooted yet and needs a bit longer. Have a look here for more detail. Don't forget to set the mower blades on the highest setting for at least the first few cuts (easy to get it the wrong way round when you're looking at the mower upside-down, particularly when it's a new one that you're not familiar with - been there, done that ).
Doncaster, South Yorkshire. Soil type: sandy, well-drained
UPDATE (in case anyone comes across this thread with the same issues / questions and wants to know how it ended up):
The soil is much firmer than it was when I first posted, so patience is key - it'll get there!
I did the first mow last weekend. I didn't trim too much off but on the whole it is very thick and lush, which I'm really pleased with. There are a couple of areas that look a bit sad (some dog urine, some I just didn't do enough to rescue after the incredibly dry week or so we had after it was laid - better sprinkler needed!), but everything looks salvageable with a bit of seed, fresh topsoil and TLC. If anyone has any tips of how / when to do that I'd be grateful - I'm guessing it's just a case of raking up the dead grass and reseeding as normal in that area?
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The soil is much firmer than it was when I first posted, so patience is key - it'll get there!
I did the first mow last weekend. I didn't trim too much off but on the whole it is very thick and lush, which I'm really pleased with. There are a couple of areas that look a bit sad (some dog urine, some I just didn't do enough to rescue after the incredibly dry week or so we had after it was laid - better sprinkler needed!), but everything looks salvageable with a bit of seed, fresh topsoil and TLC. If anyone has any tips of how / when to do that I'd be grateful - I'm guessing it's just a case of raking up the dead grass and reseeding as normal in that area?
Anyway, thanks for everyone's help!