How to work out what is killing my lawn.

Beamo

I'm intent on reversing the demise of my lawn but need help working out which of several possible causes is actually the one to focus on.  Ultimately, I either need a strategy to fix the problems, or learn whether I should take a more drastic course and dig it all up and start again.  (I'm trying the latter course on one lawn - but don't know whether this will disguise the underlying problem or fix it)

Since last autumn, there are some large patches of my lawn that have become increasingly dead and almost bare.  In some areas, the grass is now just a collection of white stumps over bare soil.  In other areas, its thinning or patchy and in yet other areas, its clear that the turf has been taken over by coarser grass varieties with thin stiff stems growing upwards or sideways.  Just to complicate things, there's an abundance of ants, wasp nests, possibly some leatherjackets and the after affects of my "fix-it" attempts so far.  Between that, the slope, the heavy clay nature of the soil and proximity of tree shade and roots, there's a little bit of everything going on!  Also, most of our street seems to have more well-mown moss than lawn, so I wonder if my attempts to get a good grass lawn are swimming against the current.

I have 4 small lawns (about 50 sq m each) on 3 sides of my corner-plot house (east, south and two on the west of the house separated by a driveway).  They are all on a west facing slope which varies from almost flat to about one-in-three gradient and even after heavy rain, water never seems to build up anywhere.

At the front of the house (west facing), the garden slopes 5 metres or so downhill, stopping at a short, thigh-high brick boundary wall.  For about a metre back from the wall, the grass has almost entirely withered away, leaving just white straggly stumps and bare earth.  This lawn has also thinned the most due to weed killing and raking but where there is grass, it seems healthy enough.  It is this lawn which mostly seems to get wasps emerging from it in the summer.

At the side of the house (south facing) the garden slopes more gently 5m or so down to a mixed boundary hedge about 3m tall.  Nearest the hedge the turf gives way to bare clay (which seems fair, given the shade) but further away, the turf is patchy, ant-ridden and generally grows irregularly.  When forking this lawn, every 20 or so forkholes seem to somehow bring up what I think is a leatherjacket.

At the back of the house (east face of the house with a west facing slope) despite living below the shade and roots of a wall of leylandii, the lawn is least bare but is heavily populated with coarser grass which seems to grow in little colonies at different rates and shades of green to the grass around.

Earlier this year, I've tried variations of weedkilling, raking, and scarifying.  More recently, I've spent a lot of time forking holes into the lawn, raking sand in, over-seeding and watering in dry spells.

Any wisdom to point me in the right direction (other than suggestions of concreting over!) to diagnose what's going on would really be appreciated.