This was raised on Gardeners Question Time last week. The person
asking the question had a lawn laid but it had dips and hollows in it
and wasn't looking well. The firm that did it had gone bust.
Two different answers were given
1. dig the lawn in and start again.
2. put a coating of top soil on top of the existing lawn to make it
level and turf over it.
Was suggested that whilst this is not standard practice it can be done.
It will depend on what your soil is like underneath and what weeds are
in the lawn. Any perennial weeds like dandilions with grow through the
new turf.

Personnally, I would turn the turf in and if necesary improve the soil,
firm, level and rake and fertilise (standard lawn laying) lay lawn and
water. water water.
You know doubt heard in the news - Chelsea's sand pitch at the momnent,
Sports turf requires a lot of water but is practically grown on sand.
For good even growth tho, there is a take home point. Good drainage.
Impeded drainage only slows the grass growth down and allows moss and
weeds in.

what turf? - how about:-
http://www.cannonsturf.co.uk/domestic.htm

Good Luck

Clifford
Bawtry, Doncaster, S Yorkshire