Okay. I dismantled the appropriate parts of the tractor to get to the PTO mechanism and, like many things in life, found the problem and it was not what I thought it would be. First, to any home mechanics thinking of doing this job or even if you need to get into the final - drive area, after you have pulled all the covers etc off, give it good degrease and wash before taking it apart. Also try to make sure you have a copy of the Parts Catalog to suit your model. In my case, the catalog covers a number of models and there are differences in the parts and the diagrams. But it is helpful. Drain the oil first and be ready for a big volume of oil to come out; ~ 27 litres in fact. The drain bung is quite large and has a magnet on it. Likewise, the hydraulics / 3 PL assembly need to be drained - 12 litres for a 404 - and the pipe fittings that are removed should be covered - plastic bags work well. To remove this it requires an 18 mm socket and the bolts are tight so don't attempt it unless you have the correct size. 3/4 inch fits sort of. Don't try it. Once the bolts are undone, the whole 3 PL assembly can be lifted clear. There are no gaskets used on any of these faces - just silicone.
The 3PL assembly is heavy. Really heavy! Minimum of two people or preferably a chain hoist or similar is needed. Once it's off, you can see the final drive gears and the PTO drive gears. And, in my case, I could see why the PTO was not engaging. The cast fork that selects either 540 RPM or 1000 RPM, was cracked - see photo. Fortunately, it had remained roughly in place and not caused any more damage. To remove the selector fork requires removing the drive gears (top shaft) which sit on two very substantial roller bearings held in place by circlips. Removing this shaft through the rear cover took some doing in my case. It'd been there since 2005 or earlier and needed a combination of pulling and some hitting with a drift. But it did finally come apart and allowed me to remove the selector fork. The gears, splines and dog clutches all seemed in good working order. The photos don't show it very well but the general condition inside the transmission is pretty good. The oil was not burned (use a good appropriate oil made for these wet - brake final drives and you'll get no brake squeals) and there were no "big bits of metal" on the magnet. The bearings are readily available 6306N and 6205 roller bearings with or without circlip grooves. I'll put the new ones back with some anti seize on the surfaces!
I also found the PTO drive outer shaft seal had been badly fitted See photo) from day one but, fortunately, there are two of them side by side so the leaking had been minimal. The only other thing that was harder than it looked, was removing the split pin and spring from the selector fork. The spring maintains pressure on the ball on the selector rail and it is a substantial spring pressure. For any of us that have had that dreaded experience of these little steel balls falling into the base of a transmission/gearbox, in this case the ball is safely retained in the selector fork. Nice one Chinese engineers.
So, I'll order the new bits, wire brush the bolts, clean the cavities and reassemble. Like most things on these little Chinese tractors, they are massively over - engineered and very simply designed. Tolerances are all over the place and I found no shims and all straight cut gears meaning very little end - loading. They do make a bit of noise when operating however but that's to be expected. But, overall, I maintain my respect for them. They are cheap and made to be maintained and fixed with little complexity and with few special tools. In these days of "right to service" issues and special tools that lie across automotive and agricutural equipment, it's kind of nice to still have things that can be repaired with relatively low skills and a farmer's tool kit.
As to why this selector fork failed, I'd suggest the operator (me...) might have had some input to it. My caution to all such operators is to be a bit gentle when selecting. Sort of feel it into gear rather than wrench it. You know what I mean....
