Can i top up the diff oil , I probably know its best to change it completely , I am a bit confused with what type and and do i need to add an additive?
I need to change the diff seal so are there any tips on doing this?

 

Use Castrol LSD oil. eg
https://www.broughtonlubricants.co.u...z-85w-90-group

or

https://www.classicoils.co.uk/axle-z-lsd

you have a drain plug on the diff bottom, so first ensure you can undo the fill plug, and once you have, drain the oil and replace, use a giant syringe or a long tube from the fillhole out through the wheelarch and up above the diff level and do by gravity.