Do you know what Tom, I would love to plaster that wall back onto the original lath. More often than not it's just a patch that comes along, but a while ago,I did a ceiling onto the original lath (the whole ceiling) because the man didn't want a sheet of plasterboard near it. There's a bit of work involved, but you end up with as near to the original as you can get. Modern plasters maybe (bonding coat/multi finish), but still lath and plaster. It doesn't matter if the laths are uneven because the thickness of plaster will take care of that. As long as the laths are in good condition, no woodworm, rot, and the nails are sound,(you can put more in anyway, galvanised). Ensure the gaps between the laths are free from debris, just poke any bits through with a screwdriver etc, or give the laths a tap gently with a hammer just to clear them, and it is always a good idea to take off the bottom 5 or 6 laths or more,depends, along the length of the wall and clean away anything that has dropped down.YOU TOM seem to have a BIG build up of muck in your wall which can cause you future damp problems, so you can clean it out this way and fix the laths back on. Another thing that I always do to old lath is give it a good wet down with a brush,over and over again from top to bottom, just brushing on water to let the laths absorb the water after years of being bone dry. Old dry wooden lath will swell up and can crack the plaster. When i'm all prepared, i'll put my levels on the wall, pva the laths,and cover the whole wall with a coat of bonding plaster and then build up my working screed, and after that, plaster the wall as you would normally. I also use a lot of jute scrim, especially on old lath, and will ALWAYS bed it into the plaster from floor to ceiling,about every 6 inches apart,right the way across the wall, breaking up the straight lines between the lath, another paranoid precaution against cracking by swelling lath, it's just my way. When the wall is finished and painted,it's as good and strong as it was all those years ago, and will last for generations.
Roughcaster.