Thanks to all for the advice, i did find a way to take the balancer shaft assemby off without all the complicated dissassembly that Audi recommend.
simply time the engine up and fit the crankshaft locking tool
You will notice that the balance shaft timing mark points upwards to a casting fillet so note this mark
the right hand of the balancer weight quadrant will also point roughly to the oil pump bolt so remember this alignment also ( i took pics)
Take off the balancer shaft cover by removing the 3x 8mm bolts and gently prising the tabs and it will come off ( or they will break like mine did)
Then undo the 12 point 10mm bolts holding the balancer shaft housing in place but leave the 6 point long one.
push the tensioner in and put a pin in the hole to hold it shut
undo the left hand idler wheel with a 6mm allen key and then move the pulley off its seat by pulling it out slightly.
you can then take the oil pump off ( watch the locating dowels )
then undo the last long 10mm bolt and the whole thing will drop down and the chain will be still attached to the crank.
when you put it back just retime the balancer shaft using the slack gained by the idler wheel being off its seat.
this gives you access to the bottom of the crankshaft.
Repairing the tooth is just the easy way of sorting the problem rather than removing the box, flywheel and dropping the crankshaft. I actually found the broken tooth in the sump! but worryingly i cant figure out what caused to to come off. At least i will be able to see if this is the problem causing the non start rather than risking loads more time to achieve the same effect.
