My lawn has not chance of recovering from years of misunderstanding on my
part, now that I am a bit wiser & more caring I would like to know is can I
lay new turf on top of the old turf, if so how much new topsoil (depth)
would I need to lay the new turf on?.

Having built a wall all around my lawn I could raise the height of the lawn
by about 6".

If it is possible to do this can I lay down matting (the stuff you buy on a
roll) before putting down the topsoil to prevent the old weeds getting
through or would this inhibit the new turf I lay?.

Mike P.