Thanks for good advice, you both confirm what i was thinking.
I have the car on axle stands front and back with the back ones under the axle, so it has the weight of the car on it now, a bit more weight would straighten the arch on the springs a little bit to make the length a bit longer but they are quite flat at the moment anyway.
i think shorten the prop by 20mm should do it....

Now, to find a machine shop that can do it, and balance it, and on a bank holiday/new year.... mmmmm

HI, listen i also made a few changes to my car a while back and this is what was done. --- five spd g/box and a alfa junior lsd so obviously a change in distance. I had it done by a private chap [does work for stock car owners,trucks etc]and i found out that he had worked for a big local co [CBS clutch and brake supplies] This chap actually told me there is a amount of free-play required think it was 20 -22 mm but mine was only approx 12 cos i WAS gonna race with it.App when you load passengers[weight] this is when the 20mm comes into play. I've have not given the shaft a hard time yet but the little i have driven it,well it's been good.I would think that you should just ask s/one that works with d/shafts etc and take it from there.I normally go to two or three places and ask ques. and then figure it out myself.First look that e/thing is in place --rear seal,nothing hindering shaft from going in,if car is on jacks measure distance from lets say the seal to rear flange and then put car back on it's wheels and measure again from same places.NOW i know what you gonna say ''how the hell am i gonna get under a lowered 1200'' well put front in and lower slowly then just stretch and try and lift it and see if there is a diff.Not really the correct way cos if you look at the front part that goes over your m/shaft it's approx 120-130mm long so if you 12 or 14 say even 16mm play there is still over 100mm left to keep you safely on the road.The way this dude explained to me was as you load weight[passengers] the free play is taken up and that is what they allow for.Dont quote me on the distancs cos that is more or less and for my use[racing] and now that i'm going back to the road i still ride alone --cos if i speed and die i'm on my own but as they say speed does'nt kill it's coming to a sudden stand-still that does

i agree with you, its too long and most likely led to your gearbox failure
you need about 25mm of your slip-yoke hanging out the back of your gearbox when its all bolted up

good thing for you is that its a lot easier to make your prop shaft shorter than it is to make it longer!

How much axial play should there be in the propshaft (free movement backwards and forwards) ?

Putting my propshaft back in today and i cant get it in, with the splined shaft hard up inside the gearbox i cant get the flange at the other end to mate with the diff flange. Its like the propshaft needs to be about 5mm shorter.

With my old gearbox in, it was very tight but you could just about manage it, but now i have put in the new one and have new mounts which are much harder there is less movement in the whole assembly.

Could it be the the propshaft is too long and this led to failure of bearings in my old gearbox?

With no play in the assembly, the movement of the rear axle could cause axial loads to come down the propshaft and into the gearbox ?

I would expect that there should be some allowance for the splined shaft to slide a little in normal use...

Car is a frankenstein with Nissan engine/box and Austin rear end, so the prop has been made up by someone in the past so i wouldn't be surprised if it is too long..

(tried hard to avoid "double entedre's in the writing of this post )