Help with a lawn which won't grow, 2nd time round!

I have a lawn which just won't thrive, here's the facts:
- Live on the sea front, it's the rear garden, 18m2, west facing. - A new promenade was being built 4 years ago and the rubble and soil from existing ground was removed and dumped in a mini car park opposite me I was doing the garden at the same time so thought it was too good to be true not to pinch the soil and use for my lawn. Did this, filled the area to a depth of 4" and sewed seeds. At first the grass looked amazing, but after a few weeks started getting patchy, very yellow/strawy and eventually areas died back. I tried adding more seed, plenty of water, no luck.
- It got to the point where I called a gardener who (for 140) took a look at it and said they'd add some top soil and fertilizer, more seed and refertilize in a few weeks. They did that and it looked better, but eventually right back to square 1, but buy this time the gardener was uncontactable! Just can't get hold of him, typical!
- Since then, we moved out to restore our home and the grass was left in a bad state, never cut (although only a few areas actually grew long enough to even warrant cutting!), but I thought I'd address it when we moved back in. That time is now and I cut it initially to clean it up and see what it's do, but no change.
- Had a skip this w/e for removing tarmac, and with some space left, I decided to dig up the lawn, and also removed a fair amount of soil (thinking maybe it's contaminated?... although it served a good enough purpose when it was used in its existing location!)
- I've dug down 2-3" and intend to buy some unscreened topsoil and turf it, but I'm worried after a few weeks or months the same thing will happen when the roots bed in?
- I did notice that after heavy rain, puddles appeared the soil is 4" over shail, could this be a factor? I also noticed dark (almost black soil when digging it up which I think coincided with a pond smell not sure if this is the soil the gardener added of if it's bad soil? And I also noticed quite a few (every other shovel load) of inch and half long caterpillar type creatures but no legs ie. like a large maggot, some were brown, some crimson red, red ones were like a cocoon!
That's all the info I can say guys, included all I know with the hope of some guidance/advice. Any ideas???

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