Stephen, I know you directed your question to David but the offset pinion is normal so stop worrying.
Mine is offset 25mm. (If I remember correctly)
The main thing is to get the three flanges square in the car as David suggests. That is the pinion and the two drive shaft flanges.
In my RMC the top mount plate was off square so that when I bolted the diff in with the tapered Jag bolts it would pull the diff out of square. ie. Looking down on the top of the diff in plan view, it ended up rotated slightly anti clockwise.
Now....it wasn't much but when you extend that inaccuracy out to the wheels it ends up quite considerable.
Cheers