May as well keep talking about this in this section. I feel like a complete dill in not having been able to rectify this problem. I know what causes the problem, I know what's required to fix it, and I WOULD IMAGINE, and I am confident in saying so, that the problem is fixed now.
So why have I not fixed it previously ? Well, the first time I was unaware why the coupling CAME OFF THE SPLINE. When refitting it to the spline, I did not realize that it was partially slipping off the square inner shaft.
So, second time round, I fitted the coupling to the shaft first, and tightened up with one of those silly star headed Allen bolts, not realizing that, as I was sliding the coupling onto the spline, I was again pulling it off the square shaft. I had established by that time the reason for the inner spline to have the ability to be pushed in and out. The other problem was a distinctive lack of tools and I could only knock the square shaft with coupling attached against the splined shaft of the box. Because of lack of space, I could not get enough of a push to drive the coupling onto the square shaft. Couplings, square shafts, this that and the other, you must be sick of this by now...I am.
But, the show must go on. So, this time I attached vise grips to the square shaft and had that arrangement abutting against the fire wall - in a sort of fashion - well, as best as possible.That should largely have stopped the shaft from moving inwards too far. I then fitted the coupling and belted it home - again, as best as possible. I then used a conventional Allen headed bolt and tightened it hard, all the time trying to keep an eye on how far the coupling was going on.
I then fitted the coupling to the splined shaft, and again tightened it hard with another conventional Allen headed bolt.
After it was all completed, I fed a thin needle through the opening split of the coupling, and I THINK that the coupling is on far enough at each end. Last but not least, I will need to paint the shaft so that it will be fairly easy to visually check for any movement. I think I've got it licked. Regards Styria