I'm somewhat puzzled by this. Digging it over to a spit's depth should have been sufficient for a new lawn, it's not necessary to dig two spits deep or anything before laying turf, so I'm having trouble trying to decide what the problem might be. During preparation, did you level and compress the soil, then rake up again before turfing?
When you say the turf took well at first, when was that, in other words, when did you lay the turf?
Second, when you say it became sparse and 'mostly off', what precisely does that mean? Did patches die off, go brown or yellow, or mouldy and pinkish white?
Third, how did you lay the turf? Did you stagger the joints?
Fourth, Did you apply feed to the ground before laying it, and if so, what?
Fifth, when you spiked and applied lawn sand, what time of year did you do it, and did you actually see water on, or in the lawn?
Sorry about all the questions, but more info is necessary to work out what might be wrong.

(hope I'm doing this right...)No not at all, very grateful for your time and help.

After digging, we levelled, possibly didn't compress well enough. Laid the turf (good quality) last October ish. After about 4 weeks, started to go brown, yellow and muddy and realised there were patches everywhere of just mud with no grass. I hadn't previously hada problem with the old lawn, just compaction but it used to be just a play area I didn't worry about that. But with new lawn thought we dig it up first. Yes staggered the joints. We didn't apply feed but we had about 4" of new topsoil brought with the turf since we hard paved an adjacent area and the level needed to rise. This topsoil contained "fertiliser for the new law", so it said. It was a good lawn company. When we spiked a applied sand (lots) it was around December - remember it was still very mild at that time then.

If you haven't noticed water lying on it, and it wasn't pooling when you aerated to apply the lawn sand, its likely the topsoil applied beneath is the source of the 'river' smell, particularly if you did not have drainage problems before you changed the lawn.
You haven't said where you are in the UK - in the south last October, muddy certainly wasn't something I'd have said because it was still very, very dry here. Did it rain a lot in October where you are?

I'm from Nottingham in the East Midlands. We had some rain - more than down south, not enough to account for the drainage problem though. Should we dig it all up again and compress down better this time and start from scratch? Some bits of grass are coming through but very very sparse. It might re-grow given time - and no rain! I think if we'd done less to it, no extra topsoil and no digging it would have been fine. Start again time???? Thanks for any help. hazel

No, I wouldn't, unless two thirds of the turf is dead, or you have terrible hollows and hummocks because you didn't 'heel' over (by walking on your heels all over it) before raking up, releveling and then laying the turf. I'd wait a while and see what happens, perhaps apply a lawn feed at the end of the month. Not a weed and feed, but just lawn food.

I hoped you;d say that! I'll feed only this weekend - I don't have any particular hollows/hummocks, though it feels soft and squidgey underfoot generally. It's around two thirds gone but more grass is popping up all the time. Many many thanks for your expert advice. hazel