What you need to check if there is oil under the box is where it is coming from. It might be the driveshaft oil seals or it could be from the case itself.
I had a leak on mine and hoped it was the seals but it turned out to be the case was cracked as a result of the diff pin rivets coming adrift, a known fault on the 020 box. when it does go it normally shows up as a whine on 5th gear as this sits higher up then the rest of the gears in the box so gets starved of oil first.
Get under the car and have a look at the dif housing on the box. It has ribs in it for strength, go round with your fingers and see if you can feel any small cracks.
hopefully it will be just the seals but it would be good to check.
