Tom wrote
....
My son-in-law has planted a lawn (not from seed) and it has done well.
one big problem. There is very little top soil and underneath is all
clay. the garden is always very wet because of very poor drainage. Even
putting in drains probably wouldn't solve the problem as only that area
would drain. Any ideas ?

He didn't think it through then. Should have sorted the drainage out by
incorporating grit/sand before he laid the lawn.
All he can try now is to push a garden fork right into the soil at a couple
of inches apart and move it around to make conical deep holes which he can
then fill with sharp sand and hope it will be enough to cure the problem,
depends how solid the clay is. The lawn will look a mess for a while but
soon green up again.

--
Regards
Bob
"Never get so busy making a living
that you forget to make a life"