300D 617.912 engine will not start

I have a two tank SVO system on my Merc with 310,000 Kms on the clock - 100,000 Kms are mine most on SVO. I have never been stumped, held up or broken down before. The engine starts and shuts down on diesel. The problem- I was away for a week and my Merc sat idle. When I came back the Merc started hesitantly (low battery). I drove about 20 Kms and the next day she would not start for love nor money; totally dead. Not even a kick from one cyllinder. I checked; Lift pump ok, replaced with electric lift pump & still no go. IP moving fuel when lines cracked. Cam timing marks and harmonic balancer marks line up ok. Valve clearance not binding, Glow plugs working, has the Mercedessource fast plugs installed. Best thing ever. The injectors were re-newed 8 months ago, Compression, 150, 200, 250, 300 psi could not get to last cyllinder, Put oil in 150 psi cyllinder and rechecked came up to 250psi, assume rings shot. I put a little petrol down the intake and tried aerostart and no hint of firing. I took it to an injection shop and they can not diagnose the problem, they assume major engine failure. There is lots of blow by even from just cranking without startup. There is a mist coming out of the exhaust pipe . While cranking there is an intermittant puff of mist out of the intake manifold (that is strange). I have compression on 3 pots, there is fuel and glow plug heat - what could totally stop a diesel like this. There is a timing cog on the IP that could skip a tooth without affecting the valve timing. Any ideas welcome. Graeme