Sorry to keep banging on about this but:
Its now in and I took her for a drive - beautiful - two washers as you suggested Joe so all good - the backyard mechanic wasn`t needed!. As per the SLS diagram I attempted to keep the two drive shafts and the gear box in a straight line but in the end I used the "hope and pray" method as I couldn`t see any method of doing this.
One thing I did notice was the fact that when I tightened up all six bolts, the flex disc ended up looking like a potato chip ie where it was bolted to the gearbox, if curved forward, to the prop shaft it curved back. Is this normal? In my mind I assumed that it would remain on a single plain ie stay flat.
Hmmm, as I write I`m thinking that maybe this isn`t so bad - the prop shaft has the centering spider and the disc attached. This is then inserted onto the shaft that protrudes from the centre of the gearbox flange BUT there is a fat rubberwasher/seal stopping it go all the way forward. When the flex disc is bolted down, forcing the centering spider up against the rubber seal, it would naturally distort the ring. Am I on the right track?