maybe 10 inches - is that OK to do? Just wondering what the grass
dying off
underneath the soil will do to the root system of any new turf
overlaid on
top. Or is it perfectly OK to do this?

Many Thanks
David

I definitely don't like the idea of levelling your lawn with soil . . . .

I can see no valid reason for not liking the idea other than a gut feeling.
The buried turves will soon die off and provide an excellent sponge for
water retention - after all the best potting compost is that made with well
rotted turves.

One thing might be a problem and that's weeds in the original grass but
localised application of weed killer with a stick type applicator should
solve it.

Regards

Geoff