The top of the concrete trench is at its lowest around the lamp post. The rest is about 10cm above the soil level. I think the engineer's drawing shows that it should be approx 475mm below ground level.
Looks seriously wrong, nowhere for the deepened edge of the slab to go, no room for drainage in the (non existent) cavity below the dpc. Get your engineer to comment, it might still be soft enough to hack some out, easier if it's slotted with a Still saw first.
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