The next thing I would check is to verify the blade is tight and that the hub is locked to the shaft. The blade rotating w/ the crank gives it the inertia to up and run.

Thanks for that tomplum. I read somewhere that the blade acts as a kind of flywheel of its own and can affect timing.

I also found out that there is a sacrificial key same as for the flywheel.

So I have inspected and it seems the blade can turn independent of the driveshaft - and there seems to be nothing locking it in place. The collar sitting above it also appears to have a piece missing! Reckon I may be homing in on the problem; i.e. the blade has been smashed. It could now be that the mower's beyond economic repair.

So, can anybody say how to remove the blade and this collar... there seems to be one bolt but how to turn it?

Post a pic? I'm not familiar with this mower itself. Normally one could just remove the bolt with an impact. Lefty loosy. You can also feed rope into the spark plug hole over the piston and as you turn the the engine the piston can only go so far and locks the engine. Itis pretty safe to do on an L head if watching out for the valves. OHV engines can be trickier. If the crank is bent, it is typically BER unless it is something special. If the mower has a blade clutch, the world changes a bit.

The 'rope-in-cylinder' thing is possible but I would rather not use the connecting rod and crankshaft for the torque that may be required. All you have to do is turn the mower up on its side with the carburetor side up (always). Rotate the blade until its pointing up and down. Using a stout piece of wood, put it on the RH side of the blade, resting on the deck. Stand on the wood block, put your wrench on the blade bolt, wrench to the left, then put your body weight on it to break it loose. I don't now exactly what kind of blade driver you have there, but it sounds like its broken. You may or may not be able to find a replacement or substitute, but generally these things are pretty generic, and you have the old one to go by.

My word, you gents are fantastic! Such detailed and sound advice- I never would have thought of the jamming the piston thing. What I did in the end was wedge the flywheel (carefully) with a bar to lock and whacked the bolt loose on the blade. So here's a pic of what I have to replace with a key to go with (like I said the flywheel key up top has a cousin down under to fix the blade, with the same idea of breaking off to save blade)

So I am hunting for this bit, a "blade shoulder" (?), it's I think off what is called a Partner P471SP, but 20 years old. When I have one I hope the machine will pull start...

The end of the crankshaft probably has a vertical groove to match a rib cast into the blade driver that acts as a key. The same system is still used, but the devil is in the details (dimensions). What happened was the blade struck a solid fixed object. The crankshaft continued to twist a little and the key rib broke out of the driver body.
The engine kicks back when trying to start because the blade was loose. The crankshaft could be bent but the sacrifice of the blade driver may have prevented that.
If you have any mower shops or junk yards (most mower shops have a 'bone yard' in back) you might find either a new or used blade driver. Its also possible you could adapt a different one like the recent Toro part from, say, a 10683. All it has to be is the right inside diameter with a key rib that fits. Then you would probably have to use a Toro blade and blade stiffener in the appropriate size (20", 21", etc.)
BTW, I wouldn't use the method of jamming the flywheel anymore. You can break a fin off or shear the Woodruff key.