Just to close this thread...
I managed to install the rear axle a few months ago, but then had to go overseas. With the axle installed, I found I had an annoying clunk when driving. Investigation led me nowhere and because I had to go away again, I gave in and trailered it to a Pagoda specialist a few hours from me. By the time we got the car there, and test drove it with the mechanic, the clunk had gone. I couldn't replicate it at all. It could be that the car bounced around on the trailer so much that someone just went into place. No idea, but it was fine.
Because the car was there, and because it was about to have it's MOT (UK annual road worthiness test) I asked him to look at my work. I was pleased that he said it had been done properly. Thanks to the forum and it's generous members, everything he went to look for, he found. He said that most of the conversions he sees don't incorporate the brake balancing valve (my eternal thanks to Joe!); many more don't drill a hole for the bigger breather (on the flat spot by the pumpkin), etc, and so it went. He did pick me up for a brake line routing that I accept was done at the end of a long day, and was pretty poor. He fixed that for me. The cars brakes work very well and she flew through the MOT (once I'd addressed a few separate rear light issues documented in another post).
On collecting the car, I drove her back home, but noticed that she sounded a little 'blowy'. That was when I found the rear exhaust manifold had cracked. I finished replacing that earlier this week, and yesterday, rather nervously, took the car out. I have messed with the linkage a little doing the manifold, because the car is hunting at idle now, but I'll tune it later. However, she runs superbly on the new rear end. The acceleration is less pronounced, but I've gained 10MPH for the same RPM around 60+ and obviously the difference increases significantly after that (I did have a GPS and wrote down the speeds, but have lost the paper. Alfred has documented it perfectly anyway using his graph). On long, steep hills, I can't really use 4th, but that's OK. I did this to make the car cruise better, and it does - much better.
To those who are thinking of doing this, I would say it's worth the hassle. Doing it on a 230 is a lot more work because of the need to change the complete brake system from drums to discs, including the lines and the master cylinder. It would be quite simple on a 250 or 280. The only major emotional drama was breaking the rear exhaust manifold. I stuffed that up but not tightening the bolts correctly to the downpipes. I probably just cranked them up, without doing each one a little at a time. Expensive lesson learned.
Thank you everyone who helped and advised along the way. This place is a great resource. I'm especially grateful to Naj, who has been so helpful off line.
I'm now hoping to be able to just drive the car and enjoy it rather than lie on my back staring at her underside.
Here are a couple of links to specific problems/questions I had along the way, which may help others going down this route (there are a zillion rear axle posts on the forum, and I've read them all. Every one contains useful info, and some of them vast amounts).
This post deals with the assembly order for the rear pivot pin:
http://www.sl113.org/forums/index.php?topic=18312.0This thread deals with the carrier bush, (through which the above said pin goes), should anyone wish to try to remove it:
http://www.sl113.org/forums/index.php?topic=12903.25This shows the compensating valve/balancing valve that is needed when converting drums to discs:
http://www.sl113.org/forums/index.php?topic=18261.0This post also talks about converting drums to discs:
http://www.sl113.org/forums/index.php?topic=10947.0I found this to be a really useful thread (not mine):
http://www.sl113.org/forums/index.php?topic=1195.150
