Differantial

[MONTY007]

What is the tool called to enable undo the inverted nut and allow me top up the diff with oil?

[bmcecosse]

Piece of square bar - and a spanner......

[philthehill]

Here is what you want on 'e' bay:-

http://www.ebay.co.uk/itm/ELORA-Germany ... 3aaeb0f31c

There are several more on 'e' bay if you want a choice.
Otherwise use the male end of a socket extension bar of the appropriate size - that is what I use even though I have one of the multi size spanners as above.

Phil