Yes there is Euan.
The plastic gear wheel driven by the worm drive shears its teeth.. the part varies depending upon the diff ratio, think the colour give a clue, 5.43 ones are usually red it seems , Aidan will know, thank goodness
The roll-pin is 5mm
The trick is to get the roll-pin out without mullering the diff casing lug and making it doubly hard. First cleean the whole area up so you can see it well (the end of the roll-pin) which will be some way into the lug. Then some WD40 just for fun, help clear some of the corrosion away, maybe pen oil might help the final push, but its really all down to dead accuracy with the right drift and good solid blow with a club hammer... you may be lucky, or as with some even on the bench, can be a swine.
Look here for the right size of drift, don't mess about with anything else, I've seen the odd diff with the lug totally mullered and the pin still in there... so follow the guide below from the ATNAC* club memoirs...
speedo drive gear replacement
Happy to send the drift I made up if you want, should work OK.
*T-shirts available soon
