Hi All
So, thinking it through from your account,
Hot or cold you don't have the 'beef' to turn it over through top dead centre.
With a tow it will run.
First make sure the air inlet is clear, then whip the exhaust off and check that is not blocked either.it won't hurt running without either for a couple of hours whilst you sort it out, should there be any doubt.
As it runs, the valve clearances must be somewhere near, they must be seating for it to run, were they not opening then it would probably not run at all.
The fuel system must also be in the ball park, as above.
You say it stops at the next compression, if it bounces back then the rings and valve sealing are unlikely to be at fault, if it stops with a thud / bang / clang then the problem may well be mechanical. was a piston striking a valve, after a short run the valve would be bent = lack of compression.
If you can slowly get it past TDC, as the compression leaks off then it's not mechanical, on that cylinder at least, continue turning slowly until the second piston reaches TDC on the firing stroke, again if it passes slowly then there is likely no mechanical interference.
If it's possible separate the decompression levers and repeat on each cylinder individually.
At this point assuming there's no nasty bangs etc. I would risk towing it into life.
Assuming it runs and all sounds in order let it warm up for a few minuets.
If it's been hard of starting for the last few years of it's service it's more than likely that it's been started first thing in the morning and left ticking over all day, this, combined with low quality 'red' diesel could well have 'coked' it up. possibly beyond belief!
This could well have increased the compression well past normal levels.
The fast and dirty answer is Glorts, water injection.
Nip indoors and steal the squirty bottle with the blue stuff the Mrs cleans the windows (or the shower, whatever) with, decant blue stuff into suitable container, for later replacement, rinse out and fill with clean tap water.
Once warm the engine is warm get it around half speed half speed and with the air filter removed start spraying it into the inlet manifold. slowly at first, one squeeze every few seconds.
Listen to the engine and watch the exhaust, there will, almost certainly be a lot of black shite coming out, DO NOT get this all over the washing, garden furniture, dog, children or anything else you don't want to the Mrs to moan about.
After a short while increase the rate you are pulling the trigger, it's more than possible you won't be able to pull it fast enough to slow the engine noticeably, if you do slow down a little and let it pick up.
If you imagine the amount of cubic meters of air the motor is inhaling each minuet you'll soon see that the amount of 'mist' you are adding to each compression stroke is minimal.
Repeat until it runs clear, this may well take some time, then drive the dumper about to clear any moisture in the exhaust and give any in the sump a chance to dry out.
Then stop it and see if it will go over TDC, with the Weetabix available!
It may or may not help but it won't do any harm as long as you build the rate of injection up slowly.
Hose the mess down, replace washing on the line, untie the dog, let the kids back into the garden and replace the blue stuff before sneaking the squirty bottle back into the cleaning gear cupboard.
Get beer and celebrate, or get beer and start the strip down.......
Cheers
Stef