Right, although i have no idea how it works, i realise what the bits kinda do now, so no worries on this thread no more, unless sumone knows the ins and outs of the ATB.
The ease with which the diff slips can be set with those washers and the splined retainers (at teh very centre of the diff) by altering their orientation. So i shall have to have a fiddle.
Oh and i rung quaife, the diff is still in production and the replacement piece i need should come to about £15, so not bad all in all.
Another update: after installing the replacement part today it ends up that the faliure and shit performance of the diff previously was down to incorrect orientation of the broken splined retaining collar during assembly previously. (hence it was forced into places it wasnt designed to go, locked and the shock load it experienced when it was unlocked shattered it).
The new retaining collar is installed correctly, and it looks like the alterations in preload actually have an effect now!!
I look forward to utilising it on track soon!